I often re-watch movies. It's something i started with rather recently on a larger scale. Something have happened with my movie taste since last summer, i somehow shifted my priorities on what makes a movie good, or should i say i learned to consider and appreciate many many more aspects of a film than previously. I own alot of dvd's and have the hdd full and somewhere back then i put on an old goldie only to discover i now thought it was absolutely aweful. Went on a bit of a re-watch craze for a few weeks there, feeling a dire need to get myself updated on which movies still are good and which aren't. The good ones are thankfully still good.
I guess it's only natural your taste changes as you become older and as your persona, interests, standards and perhaps even values change. Re-watching every now and then is definitely a good idea. You'll find lots of gems you thought were worthless.