(These are all various webfinds of Esme and her actress Elizabeth Reaser from around the net)
The movies don't reveal that much about Esme's past but she has her own backstory involving her losing her infant son and being so grief-stricken she attempting suicide. She was brought back from the brink of death by her vampire Carlisle (who would later become her husband). It sort of makes sense that she would fall into the motherly role of the clan. I think that one of the biggest draws of Twilight for its audience is the romantic overtones of the series and Esme and Carlisle's definitely have a really sweet relationship. The way that Esme cares about her husband and adoptive children is really touching and one can't help but admire her.
Ok more vampires and warm fuzzy feelings to come as this Twilight Feet Saga continues. ^_^
-KSC