Was it ever explained (spoilers)

Young girl with no experience in life. First dude that showed interest in her, went for the kill and scored. All part of Peggy's evolvement from insecure creative to confident independent.

He had the hots for her early on. I think in many cases, that alone is enough for you to become attracted to someone; the flattery that they're interested in you.

Over time you just develop a natural attraction to them as well. You like people who like you. Not all the time, but in this case, yes.

He was her first. The series opens with her getting birth control and starting her new job at the agency, fresh out of secretarial school. She makes a pass at Don but he turns her down. Then Pete shows up at her place. She wanted to be emancipated from her Catholic upbringing and the stifled future of becoming a 50s housewife.