I think there's a few reasons,
1. Writing about love lost is probably a lot easier to convey and to understand from an audience perspective since a lot of people have dealt with it (or more likely a cringy version of it where they pine after someone who doesn't return their affection).
A lot of the people making those shows at the time were broke 20 something's living in CA, and for most people at that age the most emotional thing they've experienced would be a relationship not working out.
2. Writing stories about falling in love, building a healthy successful relationship, how that love impacts decision making, and ultimately having it work out, isn't easy, especially from the perspective of a main character.
Most successful relationships in the older animated shows/movies is portrayed from the perspective of parents, extended-family, neighbors or other members of the community.
I can imagine it's difficult to write a story involving actual intimacy and how that might affect a characters actions when you've never experienced it.
Everyone can remember a moment when they were sad or depressed, how it made them feel, how it might have affected them at the time.
Less people though can remember an intimate moment with someone they love, how they felt in the moment, and whether it be a year, 5 years, or 10+ years look over at the same person they felt that for still with them, still in love.