In fact, the most important reason for their behavior is that they love too much and have a strong sense of crisis. To put it bluntly, they are delusions of persecution. Only when we care too much about someone will we often be afraid of losing. In your eyes, your lover is excellent, and you think others will think he is excellent and like him. He spends almost all his time and energy on falling in love. As long as the opposite sex appears around him, even if he is married, he will fantasize about what kind of interaction his lover can have with himself, fearing that others will take it away, and thus doubt each other in various ways. He can't afford to lose, he can't bear the consequences of breaking up, so as long as there is any sign of ambiguity, he must be strangled in the cradle, and his heart will be comfortable and at ease.
Another reason is that because of lack of self-confidence, I have a great sense of inferiority in some aspects of myself, such as appearance, work, money and so on. And think that I am not worthy of my lover, so I am afraid that he will look down on himself and even feel disgusted. I am afraid that the other person will empathize with others and have a good impression on others, so I often doubt them to express my concern for my lover.
In fact, during the period of love, doubt is a necessary state. You need to restrain your bad mentality and learn to trust each other. What's yours is yours. It's no use if you don't importune. So relax, enjoy love and don't think about it all day.