Hi Kona!
Without getting too technical, here's the standard view from the natural selection viewpoint of evolution:
At some point, abiogenesis occurs (the point at which a particular molecular structure gains (at least some of) the characteristics of life (including the ability to make copies of itself).
As a result, there are soon, quite a large number of copies of this lifeform (all identical so far). Random mutation occurs. The vast majority of these mutations are harmful, and the lifeform becomes non-viable. Every once in a while, the mutation confers a non lethal difference in the characteristic of the lifeform. If this difference provides a survival (or reproductive) advantage to the lifeform (they can survive more easily in their environment, or they can reproduce much faster than the other forms), then they will become the dominant form in that environment.
Fast forward for a bit. We're now in an environment with more advanced life, but they still all reproduce via mitosis (direct copies). Another random mutation occurs that doesn't render the victim non-viable. Perhaps it now finds that it can initiate a copying process with another non mutated member of it's type. (Most likely, this was not a one-shot event). Sooner or later, the ability is gained to reproduce by mixing genetic information from two (or maybe even more) members.
This confers a survival and reproduction benefit in the following manner: If a lifeform reproduces by division only, any change in its structure will be passed on to all future copies. Sooner or later, the error rate will become lethal (or at least non-viable in the environment, or change to something else entirely).
Bisexual reproduction however, confers two primary reproductive benefits. First, the commingling of two sources of genetic information (which is primarily identical, with only small differences) provides a great deal of resistance to cascading mutation failure (multiple reproductive sources provide redundancy). Second, the intermixing of different strands of dna also provide more room for rapid genetic variation in subsequent generations. The redundancy tends to help moderate out the more catastrophic cases. This redundancy is especially important as the organism becomes more complex, as there are many more ways to break its structure.
As a result, any species that reproduces in such a fashion is more resistant to catastrophic changes, and is still more adaptable (due to the more rapid variations).
In other words, once life attained the ability to reproduce bisexually, that strain gained the best tools for long term survival (and further evolution), to the point that only the very simplest forms of life are still asexual.
Obviously, there's more to it than that, and we certainly don't know what the particular sequence of events or triggers were. And of course, it is a hypothesis of the natural selection theory of evolution.
