I had/have this with a friend/ex friend, whatever he is..lolOur ascendants are conjunct at the end of Gemini. I've often been told, the way we come accross is very similar.
The way we talk, speech patterns (similar expressions) similar fashion.. same kind of humour. We can both talk A LOT about the most inconsequential things ever (gemini asc) and we skip from topic to topic, no correlation. Whenever there were other ppl around they thought we were very weird lol, because of the speed at which we were talking and the amount of subject twists in the conversation (it made no sense to anyone listening in
So, based on this I'd say having the same Asc accentuates the traits of that sign, the traits that you display to people.. So when you're together people see you as very similar..though you may or may not be (depending on the rest of your charts)
It also helps by adding some good will and mutual understanding between the two of you..because the way you come accross to him and the way he comes accross to you is very familiar and easy to relate to.
I think people find it easier to "trust" something they feel they know..so when someone puts up the same face (asc) you put up..it makes you feel like on some level they're "similar" to you so they're trustable (though as a word of caution, with the asc things aren't always the way they seem..so the person may or may not be worthy of your trust - have a look at the rest of his chart and your synastry)
It could also have to do with attraction I guess..because the ascendant rules "looks" so you might look alike. I always see ppl in relationships who kinda look or seem similar. I guess it depends on whether you "like" yourself and like your own looks. If you do, you'll most probably be attracted to someone who looks similar (or alternatively the opposite of that, since opposites attract..For instance, an opposite Asc that conjuncts your DC- me and my bf have this- can also be a cause of attraction)