It might be that someone installed the door wrong. It might be that the jamb or the hinges have loosened. But if you take a look, you might find daylight between your front door and the jamb alongside it. If you have more than a 1/8-inch gap anywhere between the door and jamb, it's not a difficult repair.
The jambs are the vertical wood pieces on either side of the door; the one with the hinges is known as the hinge jam. It could be that some of the screws connecting the hinges to the door or jamb have loosened. Check all the screws on all the hinges and use a drill/driver to tighten them.
Loose Hinge on Jamb
It's common for exterior doors to have at least one longer screw, penetrating through the jamb into the rough framing. This single screw may be the issue.
Step 1 Tighten the Screws
Open the door and examine the screws on the top hinge that penetrate the jamb. Look for a screw with a slightly larger slot. If there isn't one, focus your efforts on the top screw.
Step 2 Drive It
Drive the screw deeper with a drill/driver. If it won't budge, try the remaining screws. If those won't budge, continue to the next Step.
Step 3 Replace It
Remove the top screw. Replace it with a similar screw at least 1 inch longer than the original. Drive it in tight to pull the door toward the hinge jamb.
If tightening the hinge doesn't remove the gap, the jamb itself might need additional screws, or it could have become out of square due to the natural settling of the house.
Step 1 Remove the Hinge
Remove the top hinge leaf from the hinge jamb. It's OK to leave the other leaf of the hinge on the door.
Step 2 Drill It
Insert a 1/8-inch drill bit into a drill/driver. Drill two holes, one at the top and one at the bottom, inside the mortised cut-out for the hinge.
Step 3 Drive It
Insert 3-inch screws into the holes. Drive them in tight to bury the head of the screw slightly below the surface and force the jamb closer to the stud in the wall. Replace the hinge.
Deepen the Mortise
If longer screws don't do the trick, it's possible to cut the mortisse deeper to allow the door to move closer to the jamb on the hinge side.
Step 1 Remove Hinge
Remove the top hinge leaf from the jamb with a drill/driver.
Step 2 Shave the Mortise
Use a sharp chisel to shave 1/8 inch from the bottom of the mortise. The mortise should already be 1/8 inch deep, so the total depth would be 1/4 inch.
Step 3 Replace Hinge
Screw the hinge back into place. The extra depth of 1/8 inch should allow the door to move closer to the jamb.
If All Else Fails
If the door has a gap no matter what you do, it might be that the door has warped slightly. It's not likely, but fixing the warp might be the only solution.
Step 1 Pick the Spot
Examine the door. Locate the area where it's closest to the jamb; if it's rubbing the jamb that's the spot. Use a pencil or marker to draw a line on the door. The line represents the area where you will shave the edge from the door.
Step 2 Remove the Door
Remove the door from the jamb. Place it across two sawhorses. Some exterior doors have a 2-to-5-degree bevel on the edge. Check its orientation before proceeding.
Step 3 Shave It
Set the depth of a power hand-planer to 1/32 inch. Turn on the planer. Apply it flat against the bevel -- if applicable -- on the edge of the door, at one end of the pencil line. Push it forward, holding gentle pressure to shave off the edge. Repeat as many times as necessary.
After you make the adjustments, the door might not shut properly. The striker plate on the opposite jamb might now be misaligned with the striker on the door. Unscrew the striker plate, chisel the mortise in the direction needed and screw it back on.