In a relationship, both parties are responsible when it comes to trust. Some amount of trust should be freely given, as much as you are comfortable with, but that doesn't necessitate that you follow blindly.
If there is mistrust, it usually occurs because of a situation or misunderstanding between those involved which is where communication becomes important. Who is to blame depends on the situation.
I wouldn't go so far as to say there aren't ever any reasons to be mistrustful or insecure. I know I have my reasons. It doesn't excuse an overreaction, either on my part or his. It is best dealt with by communicating your feelings and, hopefully, having your fears laid to rest. If every time you try to discuss how you feel and get berated for it, it doesn't help the situation for either involved.
In my experience, bad things usually happen.
It will never be enough for me to hear that. I have to see it to believe it.
That's his choice. If, after a time, the trust isn't growing between them, it's probably best to call the whole thing off.
Many here have made very good and valid points. For myself, I know I have issues. In my mind, my fears are not unfounded. I'm not the type to accuse anyone of cheating, etc., or go ballistic without due cause, and I'm actually very forgiving and understanding. However, when someone hits a wall and decides there's no hope without actually making any effort to correct the behavior which is causing the bad feelings, I'm not willing to take all the blame.
I'm worth a fucking phone call, damn it! *cries*