As the other commenters said, something was done wrong.
I am going to ask a stupid question - they do open inward, don't they ? If not, then you need a special design door, which has sealing lips or rabbets that overlap the door stop strips to seal water out. If it opens outward and has a visible gap between the door and the frame around the outside edge of the door, then it is not the right type door.
That aside and assuming it opens inward, a bit of diagnosis you can do yourself :
There should also be a frame called brickmold or door trim (wood or fiberglass in your case) around the door, that overlaps the door frame and the siding by an inch or two on the outside of the house, unless you have a recessed door in concrete or brick, in which case it might butt up against the brick or concrete at the side instead of overlapping it - looks like this -
If you look all around the door, there should be a healthy caulk layer all around the brick mold, sealing it to the siding. There may or may not be a rain cap or flashing (metal) overhanging the top of the door frame to keep rainwater running down the side of the siding off the top of the frame.
On the bottom, there should be a metal tread or sill plate that the door bottom seals against at the bottom, sloping away from the door on the outside - like this (mixed wood and metal in this case), outside of house is at right of image -
This forms the threashold, and this piece should have been caulked on the ends and bottom before installation, and should be sealed (and waterproofed) to the framing below it.
The door opens and closes within the frame at the sides and top - the outer edge of the door's outside face should contact a door jamb or stop strip (just like your interior doors) when it closes - there should be a weatherstrip seal, usually a rubber or vinyl-coated bulb strip around both sides and the top though just foam rubber in cheaper installations. Looks like this (the stop strip/jamb is the piece labelled as 1/2" wide - you are looking down from the top at a section cut through the door here) -
At the bottom outside edge of the door it should have a door drip seal or drip shield that looks like this, which might have to be removed to install a storm door, depending on clearance between the doors -
preferably with both the protruding deflector shield (metal) as well as a flexible rubber sweep underneath. This is usually separate from the door bottom seal, which looks something like this (lots of different >
though they can be combined (not recommended as more expensive to replace when one wears out) -
By this point, you should have identified some missing piece - my guess is the rain or drip deflector and sweep at the bottom of the door, unless he totally forgot to caulk around the brickmold/door facing trim.
Hope this helped.