There was a certain bond between the two boys that drew them together. James was a Gryffindor and Regulus was a Slytherin. Frankly speaking, there should have been nothing between the boys but House rivalry and anger yet there was something there that neither boys could explain. Beneath all their snarky replies and sarcasm James felt something tingle inside him whenever he locked eyes with Regulus; even Regulus couldn't deny that feeling. It wasn't until much later that their bond changed.
Now James was an Auror and Regulus was a Death Eater yet the bond they shared was still intact.