I think he did not really get attention and affection from his family. I believe he found that through sex in a different capacity but nevertheless gave him a good feeling so he kept with it. He must of then decided that he could do it with anyone and get that feeling which is always fleeting. So he constantly is looking for that. Which is why what Diego said to him hurt him so much. I also think it is a persona he keeps up to hide his true self.
He could also just be horny all the time. There are people out there with high sex drives. Sometimes that is too much for one partner so Eric just plays around. He told Diego, when Diego was apologizing in Tai's route, that he wanted to go longer but then our character comes in.
It could also be a mixture of both.
It could also be neither lol.
Side Note:
I would love an Eric route but I feel like it would be similar to what is going to happen with Dom and Axel and I feel like we will learn more about Eric during the Lukas route.
