For Those of You Who..

For those of you who have been in a relationship for one + years, what would you say is the key to a happy, healthy relationship? My boyfriend and I are coming up on our first year together in June, and as far as I know, things are wonderful between us - however, I am worried that as time goes on he may become interested in another woman, fall out of love with me, get bored of the sex, stop finding me attractive, etc. (I wouldn't say that these are legitimate fears of mine, seeing as how he has never indicated that he would do any of these things - but then again, people usually don't  see that kind of stuff coming.)