Why are humans and almost every species on earth symmetrical?
[If we are talking about animals, there is a number of types of body symmetry: asymmetry, radial, bilateral, spiral, rotational, glide and metameric symmetries. Bilateral and radial are the two basic types, which served as main classificatory character for animal mega-systematics in the past.]

It's assumed that body symmetry is associated with general environmental gradients.

Animals with (elements of) radial symmetry have one dorso-ventral axis which echos only one environmental gradient: substrate/water column, depth etc. Besides this polarity, resources/signals/danger may come from any direction. Imagine a Hydra polyp with its foot attached to a waterweed and its tentacles surrounding the mouth on the opposite end and expecting the victim to come from any side.

Body of bilateral animals possesses the same dorso-ventral axis and yet another polarity, orthogonal to it: the anterior-posterior axis. This axis is conjugated with the gradient created by locomotion: the anterior margin of the body is likely to encounter something new first. This principle lies in the basis of cephalization: most bilaterial animals have more or less shaped "head".

Evolutionary changes in body symmetry are thus thought to be associated with changes in life-style, e.g.:


there is a hypothesis that cnidarians were originally bilateral (and many of them still show some degree bilaterality), but lost it after becoming sedentary.
non-walkers echinoderms originated from fully bilateral animals and later acquired their famed pentameric radial symmetry (were they originally fully sedentary like modern stalked sea lilies?). This symmetry, however was again distorted towards secondary bilaterality (not matching the original plane of symmetry) in endobenthic forms: sand dollars and some holothurians.
worms like nematodes or priapulids are dwellers of viscous media, similar in all directions: dorso-ventral gradient of environment in this case is absent or weak. Though the original bilateral symmetry is very clear in their body plans, signs of radial symmetry prevail in some organ systems, including the general vermiform appearance. Thus, nematodes combine triradial (head sensilla, pharynx), biradial (musculature) and bilateral (nervous system, genitalia, tail) symmetries.