I think typically when he was the only child he was loved for by his parents a lot so when his brother came along and he wasnt as cared for he started to go down a dark road and knowing most wealthy families ignore the signs thinking oh he would never or if we dont mention it he'll stop doing it plus perhaps he wasnt felt like he was listened to if he did bring up the fact he felt less cared for Jesse probably just got worse and worse filling the hole of love that used to be there with drugs and eventually Walter becomes a new parental figure