If you get the chance to live a healthy life indefinitely, though, I'd imagine that contemplating suicide might take a while. Just consider the possibilities of unlimited time: you could read everything. Listen to all the music. Watch every film ever made. Then create your own art, continue developing new science and technology. You could travel everywhere, and not just on earth. I just can't fathom someone not wanting another, say, thousand years of healthy living.
So much of what makes people unhappy or sad has nothing to do with physical health. It has to do with a broken heart. This can occur for any number of reasons that don't involve mortality. I can just envision people being curious too. You have been alive for 500 years, you've done it all, read all the books made all the art you desire to make, satisfied all of your curiosities.... save one.