Anvilhead, who posted above, made a really good post about that hesitancy by an addict to commit to a relationship, this was posted in a thread in the Friends and Family of Alcoholics forum:
Originally Posted by
AnvilheadII 
it's not that some people (alcoholic or not) don't ENGAGE in relationships, they just don't pound the tent stakes in very far. this is handy when the "evacuation" order comes - which often looks like a partner pulling their covers ON their addiction and no longer being willing to tolerate the behavior. i've heard it referred to as the "hefty bag" rule - be ready to GO with what you can fit in a couple hefty bags and hit da road.
while it SEEMS very, very personal, it is not. it's what they do. it's one of the ways they PROTECT their addiction. from work, friends, family, truth, the police - anything that could possibly interfere. trust me, nothing wrecks a good spinner than having to go to work....in two hours......cuz you been up all night.
a lot of addicts do really TRY to be IN a committed relationship...they aren't THAT defective....it's that....well, relationships are complicated and require commitment and honesty, like all the damn time, and good lord can everybody just shut up and stop bothering me, i just want a damn BEER! it becomes situational...sure i'll play along, unless it no longer CONVENIENT to ME.
Remember, he's not drugging at you, he's just drugging, it's what addicts do.