How would you know this? We don't know anything about his teen years. And I'm not defending it, I'm giving reason for why.
If it's not written anywhere, it didn't happen. If your argument here is "you don't know that it didn't happen at any point where dante's life wasn't explicitly detailed" then I could make up
all kinds of **** about DmC dante that you would similarly have a hard time proving never happened.
I like to talk about what I think happened to vergil and nero regarding their relation, but even then, I put a large "this is all theory and headcanon" disclaimer on all of those posts.
The first; He pretended like he didn't care about humanity or anyone else for that matter and only wanted to have fun and kill demons like it was one big party.
Because that is literally what DMC3 dante believed at the beginning of the game. Notice how he still kept behaving that way even when there was nobody around to see the "act"? DmC dante claims to hate being around people, but lives on a fair ground and visits strip clubs constantly. DMC dante claims to love the party, and when nobody is looking, he's still literally killing demons by partying them to death.
I mean seriously, in the opening scene he fires up the jukebox and slays a room full of demons with pizza, a ceiling fan, and pool balls. You think he's just putting on an act when he says he's all about the party? Get real.
Second; He says he doesn't have a father to further his severed ties with his own father and thus acting as if he was born with out a father and pretends as if he cares nothing for him and has no curiosity to find out more about him.
Sparda ran out on eva, dante, and vergil for reasons that nobody knows, not even dante. Because of this abandonment, eva died. Not only did dante blame his father for this, but he's constantly hounded by demons who think he's basically responsible for everything sparda did just because of his heritage. When dante constantly announces his seperation from sparda, I don't think he's putting on an act, I think he's genuinely sick of his association to sparda.
His mother died because sparda ran out on them. You'll notice he keeps a picture of his mother on his desk, but not his father. Do you honestly think he's lying when he says he doesn't care about his father?
Third; He pretends the information to open the hell gate being an important item from his mother was a minor thing and only focused on finishing their fight from before.
Well now I know your just being a trolling asshole.
Because he really didn't care about it. His reason for leaving his shop and going on this grand adventure was to see vergil, not to break into hell. He didn't decide to actually enter the hell gate until way later in the game, when an encounter with Lady changed his opinions on family, and why he needed to stop vergil.
You even quote the scene where his change of heart happens without seeming to realize what the scene actually means. At first he didn't care about these things, but because of Lady, he reevaluated what was important to him.
"At first I didn't give a damn, but because of you I know what's important now, and because of you I know what I need to do"
And despite quoting the exact scene where this happens, you still carry on like dante's motivations remain unchanged from the beginning of the game until the end.