A door that does not stay shut is usually improperly aligned at the hinges or at the strike plate and latch. This can be caused by settling of the house, improper installation of the door, or damage caused by slamming or excessive use of the door. Though your front door is heavier than other doors in your house, you don't need to call a handyman because you can fix the door yourself.
Tighten the front door hinges with a Phillips head screwdriver. Often, door issues are caused by hinges coming loose.
Shut the door. Look at the gap between door and frame along the side with the latch. The gap should be even all the way down. If it is wider at the top or the bottom, remove the screws in the hinge closest to the wider gap.
Cut an index card or heavy paper to fit between the hinge and the surface where it screws into the door. Screw the hinge back in through the card. If the door still does not stay shut, put in another section of card. This fills in gaps caused by the house settling that throws the door frame out of alignment.
Slowly close the front door. Check the strike plate and latch to see if the latch is hitting too low or too high. If there is a slight misalignment, enlarge the hole in the strike plate with a file.
Remove the strike plate if misalignment of the strike plate and the latch is large. Use a chisel to enlarge the shallow groove where the strike plate fits. Reattach the strike plate so it's properly aligned with the latch.