Let us assume, as many do, that there is such a thing as love. Let us assume that this love is an emotion, and by emotion, we mean an intangible reaction to an experience/situation. And let us assume, then, that there are emotions which are not love, but which can masquerade as love, or be a conduit of love. This is what we call infatuation.
It should come as no shock that the term is derived from the Latin fatuus--a fool, a buffoon. We act like fools when we are infatuated. Obsessing over a person, maybe (innocuously) following them around; in high school or college, memorizing their schedule. Constantly thinking about that person, wondering what they're thinking, whether they think of you. It thrives on the unknown, it is a state of chaos and exploration. It is a flooding the the senses (literally; see below) with pleasure.
During this period, our body is flooded with amphetamine-related chemicals, namely phenylethylamine, dopamine, and norepinephrine. What you experience literally is a "high." For instance, phenylethylamine is found in chocolate. Hense the connections between love and chocolate, depression and chocolate, etc.
The body will only do this for so long with a certain stimulus. A tolerance is developed. Infatuation can last a very short time, or it can last a long time. The problem with infatuation is that it doesn't last. It isn't deep, it has nothing invested in it. It is purely chemcial. If it doesn't aid in the development of love--infatuation does not become love, but helps it develope by creating an initial attraction--then the relationship is doomed to failure. By failure I don't necessarily mean that the two people involved do not stay together--worse, they may stat together simply out of a certain comfort level. A desire not to leave or be alone. In other cases, those infatuated persons usually realize at some point that that is all they feel--not love--and thus end the relationship.
How does infatuation help develope love? That's something I can't answer yet. Sometimes infatuation developes love--it keeps the person around. Sometimes that's all there is, the infatuation. Where love--mutual love between two people, not an unrequited love--comes from, though, I don't know yet.