Why do leaves of a house plant often face the window?

Many plants will reach for the light, all you need to do is turn the on quarter turn every other day or so to even out the growth.

The actual property of the plant that causes it is called phototropism. Plant hormones on the "dark" side of the plant called auxins react to the lack of light and cause the cells on that side to elongate which basically bends/pushes the plant towards the light.

Some plant tissues will actually bend towards the dark as do the tendrils on some vines. This turns the tendrils toward the dark objects (rock crevices, tree bark, the soil) so they can attach to it and support the vine.

Phloem