Did Alex love David
Difficult question.
After the death of his father, David developed a borderline personality structure. This is evident from his risk taking behavior, and his rapid attachment to Alex.
Did Alex fantasize "his best friend" or ideal boyfriend? Yes and No.
Borderline personalities can be fantastic in that they procure the feeling of being entirely and immediately loved, it can be very attractive to an insecure adolescent. The issue with borderline is that in the moments of deep attraction, everything is as perfect as it could ever be. In that sense, David really loved Alex, and Alex truly loved David.
However, borderline personality structures require a lot of prior exposure to deal with. Alex simply was not equipped to deal with the crisis, he did not know the signs of a reversal. Had he known, he could have maintained his faith in David despite the self-destructive behavior. David himself did not believe he was worthy and sabotaged the relationship and himself with it.
But also David lacked experience. A more experienced borderliner would have known to protect Alex against his impulses. He might have warned Alex that what they share is stronger than any escapade or he might have proclaimed polyamory sooner.
When a love bond is strong enough, it can endure and outlive borderline behaviors but it requires more experience.
Borderline is a true difficulty for young adolescents who will experience rollercoaster love that is then difficult to later top up with non borderline relationships. It takes time to rebuild a taste for more mundane, quiet and slow-paced relationships. What more beautiful, more intense story can Alex experience from then on? The comfort of long-lasting friendship and true loyalty.