So, speaking philosophically, why would anyone be a fatalist?
Well, the idea of determinism is a good starting place. This is the idea that everything is caused in full by proceeding events; given a certain state of events, the laws of nature will proceed in a uniform and (given enough knowledge) predictable way. This is fate enough for many people, but fatalism, in the realm of philosophical jargon, has a stronger claim to make.
Determinism claims that given a certain state of the universe, plus the complete set of physical laws, other events are certain to happen. This takes the form of a conditional statement; `if x happens, then y must happen'. Fatalism doesn't use a conditional statement; if you accept the basic argument, then y is determined to happen, regardless of any x. So, onward to an* argument for fatalism....
1. You will die on July 16, 2050.
2. Statement 1 is either true or false.
3. If Statement 1 is true, we cannot make it false.
4. If Statement 1 is false, we cannot make it true.
5. Therefore, we cannot change the future, in this case.
6. This can be generalized to any case.
Statement 1 looks like it is indeed either true or false.
If it is true, and true now, it cannot be false later, and vice versa. This may take some reflection to see; some things can be true now and false later. For example, the statement `the sun is shinning' may be true (or false) now, but it won't stay that way for long. But this is not a statement that can change; it is a statement about a moment in time, and change takes place through time. It can't be true, on Monday, that you will die on July 16, 2050, but also false, on Friday. If it is false on Friday, then of course, it was also false Monday.
Generalizing this argument for fatalism to any case is simple; you simply replace `You will die on July 16, 2050' with `you will eat Onion Soup for lunch next Wednesday', or `x will {do} y at time t.'
And there you have it. You might also want to look at Determinism, Predeterminism, and Free Will.
* Certainly not the only argument, but it's the one I understand the best, and it's a pretty good one.
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