Serial monogamy: Why do it?

I'm all for monogamy. The practice often leads to less drama and less STD-transmissions in relationships.

However, I am often wary of people, particularly my age, who have been engaged a few times, yet have not been married. I know of at least two such people, one who has been engaged three times and one who has been engaged four times. One is a guy and one is a girl.

My question to them is: Is this some type of sport or competition I'm not aware of?

It's one thing to have been in one relationship where you got engaged and it all went downhill. However, having several may be signs of something awry.

But I notice a few common traits with both of these people. They're never single for more than two months tops. Every person that they get with is a potential husband or wife. They tend to move in with their significant others after only a few months or a few weeks. Once that happens, they're thinking about the next step. They often say things like "I could see myself marrying her/him one day."

If I'm feeling all this pressure and I'm not even in the relationship, I can only imagine how this comes across to their significant others.

And yet, I question how seriously they take monogamous relationships. Any relationship should not be entered into lightly.

Therefore, I wonder if these people do it just to avoid being alone.

My question to you is what do you think of people who get engaged frequently, but never seem to follow through? Are you one of these people? If so, why get engaged so frequently?