So far, Damon's concept of love is always been that of 'a quest' where he had to work in order to win against an opponent.
He fell in love with Katherine when he already knew that Stefan had serious feelings for her, even if he was reluctant to act on them. We can assume he started flirting with her and competing with Stefan as a game, and it later turned in something more intense because Katherine was so different from any other woman he had met and he felt stifled by the life he had.
Katherine was his escape from a life he did not want, and most especially from the life his father wanted for him.
It's important that Damon, unlike Stefan, accepted eagerly Katherine's vampire, psycho nature. He embraced it to the point he accepted helping her to hunt. Possibly, he was tempted by her 'no rules' lifestyle and her power.
But loving Katherine and chasing after her ghost eventually turns Damon into someone who despises himself even more than he did before.
Disillusioned with himself, Katherine, and life in general, Damon decides he is love with Elena, the specular image of Katherine who represents as much the image of the perfect, compassionate maiden as Katherine represented the loose, monstrouse femme fatale.
Damon sees in those two very different women the image of what he wishes in himself, or of what he craves in life: power and freedom when he was human, redemption and humanity now he is a vampire.
In both cases, Damon is the stereotypical seeker on a quest to discover himself. He is figuring himself, finding his balance between his shadow and light zones by using Elena and Katherine as mirror of deeper longings.
Eventually Damon might not be happy in the kind of sheltered,quiet suburbia life Elena might offer more than he would have been in the desperately self-indulgent lifestyle he imagined with Katherine.
What he needs is an HAPPY MEDIUM.
Bonnie is that happy medium: the innocence and the power, the passion and the self-discipline and selfawareness to contain it. She is is the best of both worlds Damon has glimpsed in Elena and Katherine.
Yet he does not see it, because right now, his concept of romantic love does not correspond to the idea of a relationship between equals.
Right his concept of romantic love is wrapped in the concept of winning and fighting, both for and against something.
Because Damon has no self-acceptance and little self-esteem, he pushes himself toward women who are idols and ideals above his reach: this enables him to tell himself that if he wins the prize he values so much, then he is good enough, he is DESERVING of love and acceptance.
With Bonnie Bennett, Damon gains the possibility to develop an attraction that is less about what the other person represents and more about what he genuinely respects.
In other words, he may get to know her while he is still processing within himself the difference between obsession and genuine affection and he can discover he loves this little witch for no other reason than THE PERSON SHE IS INSIDE.
I believe genuinely that WHEN Damon will fall for Bonnie, he will realize he has never truly loved anyone else. Because his relationships with both Elena and Katherine are/were heavily wrapped into his capacity for self-delusion and his willingness to feel inferior to them, he will someday see that his relationship with Bonnie is entirely different.
BECAUSE FROM THE BEGINNING, IT WAS BASED ON TRUTHFULLNESS AND SPONTANEITY.
Conclusion: someday Damon will be ready to love in a genuine, artlessly selfless manner and his choice will inevitably fall on Bonnie.
We just need to brave up and hold on until that day.