Their Past Haunts Them
Chances are they have been cheated on, maybe more than once. Not to say that he or she will assume you will cheat too, but this circumstance will force them to question things you say to be true or false. Being cheated on makes a person question many things around them. More than just being cheated on, someone has hurt them deeply, and all they see is the possibility of being hurt again. All they have seen in their past is reasons not to trust, people who are merely temporary, and the fact that people, at their core, are evil.
To overcome this: Show them the past is behind you for a reason, so you can no longer see it.
They No Longer Believe in Happily Ever After
You can promise them forever, but all they see is for now. Fairytales are for the movies and for other people, but it seems some are not as fortunate. This might not be true, but this is what they have come to learn. You cannot trust anyone, because sooner than later there will come a time when things end. There are only two main endings to dating someone: breaking-up or marriage. Although marriage is always the goal a person with trust issues doesn’t easily see this in their future.
To overcome this: Learn to be patient, because trust takes time.
They Will Learn to Trust Again
If you are the right person, they will trust you. You will not have to prove it. You will not have to walk on egg shells to make sure they’re happy. You will have to give them time. You will have to trust them, too. Their past may haunt them, but their future will give them hope. They may not believe in happily ever after, but not every moment in a relationship is happy. Dating someone with trust issues takes hard work, patience, and a kind heart. If they pull away, give them their space. They’ll come back if they know you are sincere. Distance can either make the heart grow fonder, or make you realize that your heart is not in it.