Your dating, romance and love timeline may be governed by your brain chemicals. Nature seems to have designed us to move through three stages of love; lust/attraction, romance and committed love. And each stage is governed by a different neurochemical mix.
Lust and attraction is a powerful cocktail of testosterone, which fires sexual desire in women as well as men, and dopamine, the chemical of seeking and reward. If your lust and attraction are returned, you quickly move into romance, an excited and focused state in which your brain's reward centers are flooded with dopamine. This is the state we call "in love," and it's very different from the final stage, committed love, or true love, when oxytocin, the neurochemical of bonding takes over. Once the oxytocin bond grows strong, couples can stay together and weather the inevitable bumps in the road.
Nature's design seems to be that we stay in the first stage until we find an appropriate mate. When we do, we move into romance, the highly rewarding and exciting stage which typically lasts about two years -- long enough for the woman to get pregnant and have her baby. By that time, the oxytocin bond has grown strong, so the exciting neurochemicals die down, allowing the couple to concentrate on raising baby, instead of making goo-goo eyes at each other.
Now, everyone is different. Some women fall almost instantly into an oxytocin drunk, just knowing after a couple of kisses that he's the one, while others can never quite make it past romance. When the sparks go out, they move on.
When you understand the different phases, and how each of them can cloud your judgment, it's harder to get hornswaggled into a bad relationship.