Reassurance is a funny business.
The following is what I was told by a therapist.
When we're worried about something we always ask for reassurance and once we feel assured, the anxiety goes down and we feel relieved! The trouble is that because we're worriers, something else will crop up and again we'll ask for reassurance.
Imagine a waving line with peaks and troughs. The peaks are the anxiety. The troughs are after we've been reassured.
What's happening is by receiving reassurance, the anxiety is immediately relieved rather than letting it go of it's own accord gradually. By receiving immediate relief, the cycle keeps repeating.
In other words, we're not building our own confidence but relying instead on others to reassure us. If we build our own confidence then we no longer need reassurance because we cope with what's worrying us ourselves.
I know that sounds vague but I hope it makes some sense.