Many people say that the difficulty follows the price, and not the other way around. While this is largely true, it is also true that an increase in the difficulty will help drive the price up. Let me explain.
If I am someone who believes in Bitcoin long-term, I want to invest. But investing doesn't have to be just buying Bitcoin directly - it can also be buying miners that produce Bitcoin. A rational investor will look at both options:
1) Should I buy Bitcoin directly OR
2) Should I buy a Bitcoin miner that can turn my USD today into even more BTC down the line than what that USD would buy me today?
All the way from when BFL announced their ASIC products up through July-ish 2013, mining looked to be a potentially better option than simply buying BTC directly. It looked as though an investor could potentially receive more BTC through mining than they could through buying BTC outright. But now? Now, mining is a pretty terrible investment (though it gets better with the price increase). Investors looking to acquire BTC will simply buy BTC outright instead of buying mining machines for it. And that will drive the price up, whereas buying miners will not.
Conversely, if the price goes up significantly without a corresponding increase in difficulty, mining will again look like an attractive option to invest in.
In this way, difficulty and price are most assuredly linked both ways, and keep each other in check.
I told the exact same thing to my brother today (he feared a crash and he asked my why the price went up so fast and if he should sell, he has only a little over 1 BTC and was worried for his initial investment of 100 EUR :p )