The way your mind works can sometimes be a bit much for other people to handle, especially for people who don't know you well. You have a way of probing into others' feelings and motivations, deeply and relentlessly, that can make them very uncomfortable. And when you're doing this to someone you don't know well -- such as on the first date with someone new and special -- it can come across as overwhelming, or even inappropriate.
Sometimes people want to hold their cards close to their chest; they have their own, valid reasons for keeping some things private, and it's not always the best idea to push and push at them until they open up, especially if you're not willing to reveal your own. Though you want to know what makes other people tick, you're rarely willing to reveal your own motivations; you want to maintain an air of mystery while exposing others' mysteries to the light. In a romantic relationship, your urge to get to the bottom of things can sometimes verge into the territory of outright and often unfounded suspicion. Sometimes, it really is best to take things at face value, such as your lover's word.