There are really two types of people in the world: those who strive to help you feel better about yourself, and those who build you up and then tear you down. If you spend too much time with the latter type, your self-confidence will be a roller-coaster that makes it more difficult to get back “up” again each time you are pushed down.
The narcissists who manipulate you in this manner are only out for themselves. They literally have no conscience.
Sad story
I knew of one such person when I was in college; he made me feel fantastic for the first few dates, and then it all went downhill from there. He ultimately left me without saying goodbye, started dating someone else, and came in to work every day (we both worked for his father) asking for advice about his relationship with her. I finally moved away, started a new life, met the man who was to be my husband and didn’t look back.