Doors warp with changes in climate, so good weather-stripping should be compressible to take into account small warp differences between day and night, and adjustable to take into account a larger warp movement that starts in the fall and returns in the spring.
Doors, like windows, usually have air leaks through the crack between the rough and finished frames. The trim should be sealed to both the door frame and the wall -- or remove it and seal the gap behind it if this was not done at the time of construction. Just stuffing fiberglass into this space will do little good, although for many years that was the norm. (search keyword "sealing" for the title " WHERE SHOULD I SEAL AND HOW?")s