Ross Geller is a toxic person, and almost everyone around him was just as toxic for supporting his creepy fixation on Rachel.
He had a crush on her in highschool, which I'm assuming developed without him even truly getting to know her, and continued to put her on a pedestal for literal decades afterwards.
He repeatedly got into new relationships, often flaunting them at Rachel to spite her for various reasons, then acted like a child when she dated anyone that wasn't him.
Even when he starts dating Charlie, who had just broken up with Joey, he expects Joey to be fine and then can't handle that Joey is with Rachel. It gets ridiculous to the point that Joey offers to break up with Rachel to spare Ross's poor feelings.
And no one on the show sees anything wrong with any of this.
Phoebe and Monica think that Ross and Rachel are destined to be, and deliberately overlook the most skeevy, creepy things Ross has done/said.
He literally asks her to throw away her dream job just to stay with him.
She could've done so much better, her friends should have encouraged her to do better, and I'll forever be salty about the fact that she stays just for Ross.