Immature people deal with conflict or disagreement through aggressive means. They misunderstand empathy and compassion as weakness. They try to establish their superiority and authority over someone else by demeaning or insulting them. Immature people react very negatively to perceived rejection or abandonment. They are very amicable when things are going their way, but as soon as they cannot control something, they are incapable of acting gracefully under pressure. They try to turn the tables around by acting like they rejected you, rather than accepting that everyone is entitled to their own opinion. Immature people do not leave their comfort zones, and resent those who do.
Then, all of a sudden, it happens. Your BFF starts dating that person that you had already expressed interest in. What gives? It can easily leave you feeling hurt, confused, betrayed, and angry all at once - and understandably so. Not only are you dealing with the fact that someone else is dating the person you like, but that someone is your best friend.
Don't jump to conclusions about why they still live at home. They might still be hanging out with roommates that happened to also birth them, but not always for the reason you think. They could have a great job and just be saving up so they can buy a place, not rent. It doesn't mean they want to stay there forever. It's not like they love being there or that something's wrong with them for being there. They probably want to move out as soon as circumstances allow. They're not a basement-dwelling weirdo.