Replacing light bulbs isn't the most fun thing to do, but it's easy to know when to do it. You turn on the switch, the flash scares you to the point where you nearly pee yourself and you go get a new bulb.
With other things in our lives, it's not so easy to tell when they need fixed. And often, after they burn out, it's too late.
Don't wait until your friends abandon you, your family despises you or your boss fires you before you make changes. Evaluate your relationships on a regular basis and make sure you're making small repairs along the way. Once those bulbs burn out, it's sometimes too late to fix them.