Sam Winchester was the resident-Casanova in Supernatural. Given how intense his demon-hunting schedule was, it was remarkable how he would constantly find time for his romantic pursuits. His charming nature and good looks certainly helped as most of the potential partners did the approaching.
Despite Sam having an easy time in matters of courtship, true love eluded him for most of the series. His relationship always fell apart due to tragedies brought about by supernatural elements. At times, it was Sam's own reluctance to fully commit that ruined things for him.
Ruby
Ruby knew Sam before they first met. She was aware that Sam was the last special child targeted by Azazel. In Season 3, she saved him from a demon attack and expressed her interest in him. She promised Sam a couple of things and he believed her with little question. But it turned her she was only using him for her own agenda.
When Dean went to Hell, Ruby briefly became Sam's hunting partner. Even though their sex life was good, Ruby brought more harm to Sam than good. She made him addicted to demon blood and influenced him to be a darker version of himself. Interestingly, actor Jared Padalecki (Sam) is married to Ruby actress Genevieve Cortese in real life.
Becky
Becky was a staunch fan of the Supernatural books written by Chuck. She was thus star-struck when she met Sam. She tried to flirt with him a couple of times but he didn't reciprocate. However, he tried his best to tolerate her. When she began dating Chuck, he pretended to be hurt but in truth, he was very relieved.
In Season 7, Becky put Sam under a love spell that made him obsessed with her too. She was quick to marry him and was more than glad to show him off. Unfortunately for her, the spell wore off. Sam divorced her but felt guilty about it. He assured her that she would find someone she deserved in the future.
Sparrow Jennings
During Sam's soulless state in Season 6, he met Sparrow and had a brief relationship with her. She was a hippie lady who was part of a cult that believed in UFOs. Around that time, Dean had also been kidnapped by fairies but Sam ignored him in order to be with Sparrow.
Sam told Sparrow about Dean's disappearance and she was very fascinated by it. She went back with him to his motel room where they had sex. Dean later escaped and headed to the motel where he found Sam and Sparrow in bed. He was angry at Sam for refusing to save him but he didn't care. Sparrow tried to be nice to Dean but he was dismissive of her. She and Sam would then lose .
Velma Dinkey
Sam dated a Scooby-Doo character, too. In Season 13's episode "Scoobynatural", Sam, Dean, and Castiel were sucked into the animated world of Scoob-Doo where they were forced to solve a monster mystery. While there, Velma became attracted to Sam. She commented about his "broad shoulders" and flirted with him as Dean was busy trying to impress Daphne.
The two were never going to become a proper couple since Sam was human and Velma was a cartoon character. However, Velma wasn't giving up. She went for the kiss, surprising Sam. Dean hilariously told Sam he "should've known Velma was good to go." Unfortunately for Velma, Sam had to go back to the human world.
Amelia
Sam was in a relationship with veterinarian Amelia when Dean was stuck in Purgatory. He met her when he brought a dog to be treated but her impression of him wasn't a good one. At one point, she even believed he was stalking her, which wasn't the case. Eventually, the two fell in love and began dating.
Everything seemed good until Amelia's husband Don, who had been presumed dead, showed up. She decided to give him a chance, leaving Sam disappointed. Amelia had a change of heart again and considered leaving Don for Sam. However, Sam had lost interest in her. He chose to stick to a life of hunting instead.
Amy Pond
Sam met Amy as a teenager when he recused her from a group of boys that were trying to attack her. They shared a kiss and fell in love. He soon found out she was a Kitsune but spared her when she killed her mother in order to save him. Amy wanted Sam to flee with her to start a new life but he chose to focus on helping his family.
The two met again in Season 7 when Sam was investigating a string of killings. He spared her again when she was told him she was only killing for the sake of her son. Dean wasn't as sympathetic as Sam so he killed her. This caused a temporary rift between the two.
Lori
In the Season One episode "Hookman," Sam and Dean investigated a vengeful spirit that killed its victims using a hook. Two of the victims happened to be Lori's boyfriend and her roommate.
She and Sam bonded over the recent losses of their romantic partners as Sam had just lost Jessica. Lori was willing to pursue a relationship with Sam and Dean was on board, too, opting to stay in the town a few more days so the two new lovebirds could spend more time together. They shared a kiss but Sam pulled away, saying it was too soon for him to move on. He then left.
Madison
Before she met Sam in the Season 2 episode "Heart," Madison was bitten by her neighbor Glen who happened to be a werewolf. Sam fell for her and was eager to conduct an experiment on her that would turn her back into a normal human. When he believed the experiment had worked, he slept with her.
Sadly, Madison turned into a werewolf again. She had no control over her state as she would only turn and harm people when she was asleep. She thus begged Sam to kill her so that she wouldn't hurt more people. Sam had to do it but was devastated by the loss since she was the first person he had loved since the death of Jessica.
Sarah Blake
Sarah was the daughter of a man who was selling his house. She met Sam in the Season 1 episode "Provenance" in which the Winchester brothers were investigating a haunted painting in the house. She decided to help them with their ghost-hunting mission because she felt she had let the clients down by selling a house with a haunted painting.
After the mission was over, Sam stayed in the town a few more days to spend more time with her. Dean even told her to "marry that girl" but their relationship fell apart because of Sam's busy schedule. The two would reunite years later when Sarah was married. Before they could rekindle their romance, Sarah was killed by Crowley.
Jessica "Jess" Moore
Jessica was Sam's first lover in the series. She was introduced in the pilot where she appeared to have a very blissful relationship with Sam. He was ready to marry her before Dean showed up to tell him about their father's disappearance and the mystery of the Woman In White. Sam decided to help Dean out briefly but while he was away, a tragedy happened.
When he returned home, he found Jessica pinned to the ceiling with her abdomen cut open. She then burst into flames, dying the same way Sam and Dean's mother died 22 years earlier. It took time a long time to get comfortable with dating other women. On a brighter note, he avenged her death.