It isn't that he intentionally means to not trust them; it's that he doesn't even think about it. Trust isn't something Cloud has ever given a thought to, mostly because he has known for most of his life that trust is something that should never be given.
He sees the hurt in Tifa's eyes when he turns away from, walking into something dangerous without telling her, or anyone even; he sees the hurt when he brushes aside her advances of friendship, when he refuses to let anyone inside when he is suffering something.
There are different expressions in his friend's eyes; a lack of judgment and something not quite, but almost akin to, pity, the desire to get past that lack of trust. There's no understanding, because the one who would understand isn't around (and sometimes Cloud wishes he was; to not feel quite as alone in the situation would be nice, if possible, and that thought alone twists in his gut. He knows his friends don't want to disturb him even more, especially with something as sensitive as his trust issues).
Zack had been the one he trusted, the only one, it feels like. Because he had no choice (it's hard to not trust someone when they drag you around, unconscious and as helpful as a baby) is what Cloud tells himself, but he knows it's not true. Zack he trusted because it was Zack, and his friend.
And Aeris he had trusted, maybe even loved, but those feelings were more complicated than anything he cares to think about. Aeris had inspired trust in so many of them and kept them going even after all was said and done. Sometimes, when the alcohol in the drinks Tifa pours for him is just a little too strong even for his metabolism, he wonders if things could have been different; if, maybe, Aeris could not have been so trustworthy. But those thoughts lead him down a trail even he doesn't care to wander down, and even when not entirely sober, Cloud can push them aside (maybe not as easily as anything he's ever done, but there have been harder things, like watching her die and remembering Zack's death and the final killing thrust of his sword into Sephiroth-)
Sephiroth, though, Sephiroth he never trusted; Sephiroth he admired and idolizied but never truly trusted and he refuses to think differently.