theres no reason why you have to get it cut, cutting wont actually make it 'grow thicker', hair grows from the scalp so cutting the ends will not make any differance to how it grows out. people believe this because when it is cut blunt it appears thicker as all the wispy ends have gone. does that make sense?
you just do whatever you want to do. imo if you like it then just leave it! there is plenty of time for haircuts!