How to fix a door that always swing open?

I fix a lot of interior and exterior doors and the majority of the time the problem is related to two items. One, the hinges are loose, adjust/tightening them helps. Sometimes you may have to replace the screws with longer ones. Note: Most doors are butted and shimmed to doubled 2"x4"'s, you can use a 4" all purpose coarse thread screw to replace the existing screws (Don't set you electric drill too tight, as it can over tighten the screw and damage your moldings.) Two, The weatherstripping is giving you issues.