IMO, the whole "distance makes the heart grow fonder" thing is a load of crap. I personally think the idea only really applies to people you dislike or can't really stand in the first place - the same way that you can have fond memories of a favorite childhood movie only to watch it again as an adult and realize it's total crap. The longer you're away from someone that irritates you, the more you start to forget why they're so irritating in the first place.
When you love someone, be that your boyfriend, your best friend, or your mom, and you're apart from that person, for whatever reason - of course it's going to hurt. Distance hurts. As far as I'm concerned, in a LDR like so many of us are or were in, if there ever comes a time where that distance *doesn't* make your heart twinge, even if you're relatively happy in your day-to-day life and the twinge is just a bittersweet reminder of what you know you're missing (like when you've been married for twenty years, as in MrsPink's case), *then* you're in trouble. That's when you know it's time to re-evaluate the relationship and figure out if it's really what you want after all.