When pressure is applied to the outside of the toothpaste tube, toothpaste squirts out the opening in all it's gooey glory. In the same way, whatever is in you, whether it be the good, the bad, or the ugly, is what will squirt out when life puts you in the pressure cooker. After thinking it over, I decided there were a few more good analogies in the toothpaste tube of life.
Take the cap off the tube so it doesn't explode under pressure
Life is going to put pressure on you. You might as well put that pressure to good use by writing about it. The three benefits of this are that you'll have some good material that you can draw on later if you need it, that you'll get better at writing (can't beat that), and that it will provide a private way for you to release your frustrations. It's kind of like a pressure release valve for your internal toothpaste tube. An added bonus is that your friends and family will be relieved that they no longer have to wear safety goggles around you.
You can control when and where the toothpaste squirts out
You might not feel like you have control over life, but you do have control over your writing. Harness your internal exasperation (or exultation) by grabbing a pencil and writing with that intensity instead of wasting it. As they say, write hot; revise cold.
Even when you think it's totally empty, there's always something left in the tube
Ever get to the toothpaste tube late at night after the rest of the family has managed to eek out the last of the toothpaste, only to find there isn't a replacement tube in the closet? All of a sudden, you are highly motivated to put on a little extra pressure on the tube to get what you need, because there is no way you want to climb back into your clothes and drive to the store.
Writers don't have the luxury of waiting for life to provide the pressure to write. A writer reads and writes. Lots. Just do it.