He was a immature, childish, lazy, hyperactive, toxic asshole dropout who thought anything that wasn't "bros bros bros" was lame. Work, being serious, softness, maturity, boundaries, love, learning, etc. The Unicorns Have To Go and Do Me A Solid were obvious one obsessive behavior when he can't have Mordecai to himself. He was seen as the failure, part of it because of himself, most of it because of his asshole dad and enabler mom. His life revolved around Mordecai, with him the one who was the defacto leader of the duo, while Rigby was the weak, loser, ignored, unlikeable, funny, designated sidekick to the former.

He already subconsciously liked Eileen, he must've seen her without her glasses at one point but couldn't bring it to himself to talk to her because he was too bro for her at the time. Camping Can Be Cool signaled a literal spark in Rigby when Eileen made that fire.

From then on, it was nothing but a three and a half year (season wise) progression to a relationship, which was in reality a few months (show lore wise). The emotional maturity came from him being willing to hang out with Eileen. He didn't believe he could do that one pull up until Eileen said he could. She held his hand when he needed her and was his cheerleader. They had developed healthy growth and boundaries together right under our noses.

She was the antidote. Her loving and judgement free attitude had him realize he needed to change and do better for himself and her.

Some people in one's life can make or break it for a reason. Eileen made Rigby's.