Why don't we trust our mates?

Why is it so hard for men and women to trust their mates when they're in relationships? If we see the numbers of members of the opposite s*x in their phone, we assume they're cheating. If our mate doesn't pick up the phone on the first ring when we call, we think they're cheating. Is it that we're actually self-conscious and are afraid that our mate will find someone else? Is it because we have or have considered cheating, so we assume that our mate has too? What is the reason for us so quickly jumping to the conclusion that we're being cheated on? And why do we let our friends put ideas into our head about our relationships. They give their opinion about our relationships and we automatically treat their words like scripture from the Bible. We stop trusting our mate based upon he say, she say. Trust is an important ingredient in a successful relationship, without it your relationship won't last. No one wants to be continuously accussed of cheating. This will either make them want to cheat or break up with you do to your lack of trust. So, why don't we trust our mates?