All men seek happiness. This is without exception. Whatever different means they employ, they all tend to this end. The cause of some going to war, and of others avoiding it, is the same desire in both, attended with different views. The will never takes the least step but to this object. This is the motive of every action of every man, even of those who hang themselves.
Happiness, therefore, being found to be something final and self-sufficient, is the End at which all actions aim.
So at least two great Western philosophers believe that happiness is the cause of every human action. Aristotle's argument, which is quite thorough, seems difficult to refute.
Are there alternative theories about how people are motivated?
To give a bit of background: It seems certain that Pascal is expanding on Aristotle in his thoughts. And Aristotle isn't exactly using the word "happiness" as we know it in English. Here's a translator's note about the word:
This translation of εὐδαιμονία can hardly be avoided, but it would perhaps be more accurately rendered by ‘Well-being’ or ‘Prosperity’; and it will be found that the writer does not interpret it as a state of feeling but as a kind of activity.
Finally, Aristotle bases his entire system of ethics on the simple idea that all actions aim at happiness, so it's critical that we establish this principle.
Clarification: Some answers suggest that the idea is tautological. Aristotle acknowledged the idea immediately after the quote above:
To say however that the Supreme Good is happiness will probably appear a truism; we still require a more explicit account of what constitutes happiness. Perhaps then we may arrive at this by ascertaining what is man's function. For the goodness or efficiency of a flute-player or sculptor or craftsman of any sort, and in general of anybody who has some function or business to perform, is thought to reside in that function; and similarly it may be held that the good of man resides in the function of man, if he has a function.
And a bit later:
What then precisely can this function be? The mere act of living appears to be shared even by plants, whereas we are looking for the function peculiar to man; we must therefore set aside the vital activity of nutrition and growth. Next in the scale will come some form of sentient life; but this too appears to be shared by horses, oxen, and animals generally. There remains therefore what may be called the practical life of the rational part of man.
The translator notes on the word practical:
‘Practice’ for Aristotle denotes purposeful conduct, of which only rational beings are capable.
Finally, Aristotle asserts:
Moreover, to be happy takes a complete lifetime; for one swallow does not make spring, nor does one fine day; and similarly one day or a brief period of happiness does not make a man supremely blessed and happy.
Pascal expands this point by saying that people choose different methods, not because some like to be happy and others don't, but because all are pursuing a better life. Over a lifetime of bad decisions, suicide may seem the happiest choice. For both men, the context of the idea is ethics.