I almost learned this the hard way a few years ago. I was in my late 30s, and handled going to the doctor the way I always had since reaching adulthood -- which is to say, I didn't bother with it unless I was feeling out of sorts. Routine doctor visits seemed like a waste of time. Why sit an hour in an office to have somebody tell me I was doing OK, which I already knew?
Then my glasses broke, so I went to my local optician's to get a new pair. They did a routine eye exam to make sure my prescription hadn't changed, and in the process noticed something odd about the little blood vessels in my eyes. That led them to take my blood pressure, which turned out to be high. Like, really high. The kind of high where the person giving you the test looks you in the eye afterwards and asks you, entirely seriously, how you are still alive.
I was a stroke victim waiting to happen. (See http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/W...) And I had no idea. And would have continued to have no idea, right up to the moment when my circulatory system strangled my brain to death, if my glasses hadn't broken.
I went to the doctor, they gave me some pills to handle the immediate crisis, we changed my exercise routine and diet a little to bring my blood pressure down for the long term, and now I'm fine. But I don't skip out on routine doctor visits anymore! And neither should you.