Markets decide what something is worth. Its a collective decision of many people, not what one person "feels" its worth. The supply of bitcoin is fixed at a specific rate, but demand keeps going up, which increases the price.

If people weren't willing to pay the price of bitcoin, they wouldn't. Enough people feel the price of bitcoin is at value to buy at the price others are willing to sell. Price setting is one of the major advantages of a free market.

There is still a large amount of value stored in other currencies and investments, the potential market value of crypto-currencies is in the 10s of trillions of dollars. Perhaps more. Its value definately hasn't topped out. Whether or not bitcoin has outlived its value in relation to other cryto-currencies is another debate. It seems it may be due for a correction, but the trend will continue up for some time. I would still diversify for safety.