Out of all of his brothers why did it have to be him?

The two of them were Opposites, though they did had their similarities.

Both of them were loners, preferring to be alone to do their own thing. 

He wore his heart on his sleeve, hidden in plain sight but only seen by those closest to him. It was a defence mechanism, to try and stop himself from getting hurt emotionally. Raph was loud and boisterous, and Donnie was quiet and peaceful.

Brains and Brawn. 

It was wrong to fall for him.

Donnie just hoped that Raph was feeling rebellious.