The other answers given so far show examples of non-altruistic foundations of ethics. But your question asks for something much more interesting: why is altruism the default? And the place to go for this is Nietzsche. Not necessarily because he is right, but because his thoughts on the matter are the most original and the deepest.
On the Genealogy of Morals can be read as a straightforward answer to this question---how did we get where we are? (You could also look for metaphysical or epistemological insights, or philosophy of action, or psychology, or...) And his answer is pretty straightforward.
His task is to show the origins of the altruistic impulse, so if he posited that they were just basic parts of humanity he would not really be doing his job. (Besides, this is not plausible, as he shows in OGM 2:3 and 2:4; we used to be far crueler than we are now.) He examines a hypothesis popular in his own time, that morality grew from the recipients of kind acts calling them good, and finds it lacking. It does not correspond the the actual traits of early morality, and it is implausible the strong sorts, the ones doing the good acts, would want or need the praise of the weak. Instead it originally came about as
'[T]he good' themselves, meaning the noble, the mighty, the high-placed and the high-minded, who saw and judged themselves and their actions as good, I mean first rate...
(OGM 1:2). A picture I always found useful was of a very strong man picking up a rock, noticing how heavy it was, noticing he could pick it up anyway, and saying, in a caveman grunt, "Hah. Nice."
Nietzsche sets up an opposition between essentially these primitive higher types, or masters, and those they subjugated, their slaves, who were forced to be creative to survive. (This is doing a great deal of injustice to his account; you should read the book.) The higher types were a lot like lions or birds of prey---neither of which are noted for their altruism. Altruism came about because while
[A]ll noble morality grows out of a triumphant saying 'yes' to itself, slave morality says 'no' on principle to everything that is 'outside', 'other'...
(OGM 1:10) which manifests as follows:
There is nothing strange about the fact that lambs bear a grudge towards large birds of prey... [T]he lambs say to each other, 'These birds of prey are evil; and whoever is least like a bird of prey and most like its opposite, a lamb,---is good, isn't he?'
(OGM 1:13). The triumph of slave morality came about when Christianity conquered the west---Christians being the inventors of slave morality. (Actually here Nietzsche says Jews but he's pretty clearly not talking about actual Jews; he has nothing but praise for the Old Testament.) This is probably the weakest part of the argument and some later commentators take the slaves' revolt in morality as either semi-mythical or tied to capitalism.
Now unless you are already inclined in a Nietzschean direction this should all seem pretty implausible. So much of his argument depends on how it all hangs together as a coherent story, and I cannot present very much of the story here at all. So I'll leave you with a motivating example:
The praise of the selfless... is certainly not born out of the spirit of selflessness! The 'neighbor'praises selflessness because it brings him advantages! If the neighbour himself thought 'selflessly', he would reject this decrease in strength, this harm for his benefit...
(The Gay Science 1:21). This might seem to be in conflict with the earlier discussion of good as not originally coming from the recipients of good acts, but it is not. This is not supposed to be the origin of good overall; good as a concept already existed. It was co-opted for this purpose.