When a person thinks that his or her partner is attracted to someone else then the person might not trust their partner again , when you are worried that or thinking that your partner is attracted to someone else , you might begin to get so jealous and when the jealousy is not controlled it can then make you to begin to have less trust for your partner or you might not even trust them anymore especially when towards the opposite sex.
Another thing which could make a person to begin to doubt their partner or stop trusting their partner is when there is not enough communication with their partner , when you and your partner do not communicate well , it will make you people to know nothing or just know very little about yourselves and when you both do not know each other well enough then of course there will be lack of full trust in such relationship..
Another case which might make a person to stop trusting their partner is if there is a past event which involves cheating , if your partner have cheated on you before then of course it will be difficult for you to trust them again because they have already betrayed your trust and so it will not be easy to trust them again throughout the relationship