No one can deny that having your own time machine would be pretty awesome. Who wouldn't jump at the opportunity to go back in time and hang out with some of the most important figures in history, like Abraham Lincoln or Tyrannosaurus Rex?
Unfortunately, even if you survive the accidental rewriting of history and/or destroying the space-time continuum, time travel will also make your own life pretty shitty as well.
6You Die Faster
To borrow from Professor Rufus of Bill and Ted University, "No matter what you do, no matter where you go, that clock, the clock in San Dimas is always running."
"However, time will never run out on these sunglasses."
So you get in your DeLorean and you leave 1985 to go on an adventure in the past. Say it takes you six months to accomplish your goal (ie, nearly making out with your mom) and when you're done, you go back to your own time. Maybe you go back to the very moment you left.
But you are six months older. There's no way around it. The time machine can't adjust your age backward--if it did, it would be altering your brain at the same time, wiping out the memories of what you experienced. No matter how many rejuvenation clinics from the future you visit, you will always age along your own timeline just as certainly as Marty did throughout Back to the Future.
Poor kid aged five years in one damn weekend.
When you get to the core of it, traveling through time is pretty much a deal with the Devil. Yes, it will enable you to save John Connor and get to Muggle Studies on time, but it will shave several months to several years off your life depending on how much you abuse it. These are precious moments from your twilight years that you will never get back: one last weekend with your wife, your granddaughter's graduation and the inevitable cloning of John Candy.
In short, when you've managed to hit age 89 in seven years and you're lying on your deathbed, will it really have been worth seeing everything time had to offer just to miss out on your own life? Sure, things may suck now, but you have no idea what it's going to be like in the future. By rewriting it all, you may just end up missing out on your own God-authored happy ending.