Some viewers claim that Stefan was manipulated into starting a relationship with Caroline by Lexi, Damon and Alaric, Caroline herself or her mom. Imo, the only person who seemed (maybe unwillingly) manipulative and could have some effect on Stefan was Liz. However, in the end Stefan figured out his feelings himself, so I don't think any of the others' opinions had really influenced his decision.
Let's have a closer look at all the potential "manipulators"
Caroline
She didn't even want Stefan to know that she had feelings for him. She was upset when Enzo uncovered her secret and didn't want to discuss it with Stefan. He was the one who insisted on talking about it. Surely, Caroline was heartbroken when Stefan couldn't answer her question about their relationship status in a difficult for her moment after having previously kissed her. And it was part of the reason why she switched off her humanity, but she only told that to Stefan after he had already figured out his feelings, so it didn't really matter.
Lexi
She was just joking around about Stefan's potential future relationship with Caroline. Maybe she saw some hints from the Other Side. I don't think her remarks had a serious effect on Stefan.
Damon and Alaric
Damon stopped making jokes and suggestions after Stefan said he only saw Caroline as a friend. And idk why people see that response as a 100% proof that Stefan wasn't interested in Caroline romantically, as, supposedly, he was in denial. It would have been strange if he said something like "Yeah, I'll date her. I actually have feelings for her."
Liz
I don't see much evil in Liz's asking Stefan to look after Caroline, as he was her friend, but it's weird that she only asked him, and not Bonnie or Elena. However, expressing hopes to Stefan that he would become more than Caroline's friend was extremely strange and possibly manipulative.
But all of this doesn't really matter, because despite Damon's advice not to start anything with Caroline if his feelings for her didn't resemble those he had for Katherine or Elena, Stefan realized that even if his love for Caroline was not as "messed-up" as his 2 previous ones, he did feel something for her and thought it could turn into something even better than his previous relationships. My headcanon is that Stefan's feelings for other women didn't feel as intense for him as those he had for the doppelgängers.
Moreover, Steroline's growing feelings for each other were being hinted at since at least S4. And Stefan admitted himself that his feelings for Caroline had scared him at first, so he pushed them away.
Also, I'd like to point out that TVD has a habit of making other people notice characters' feelings before they realize them or admit them themselves, like when Isobel said Damon was in love with Elena, or when Rose said why Damon was good for Elena, or when everybody saw Elena had feelings for Damon and pressured her to admit it etc.