<human>: My wife and I are both in our late 30s, married for over 16 years, with 2 kids under 10.  I have a friend at work who is a woman, mid-thirties, and we've become close friends over the years.  She is also married, and her, me and another woman run ~5X per week, so we see each other pretty frequently.  Her husband and I are also friends - he's cool.

The frustrating part is that I can't figure out this woman.  Sometimes she is my best friend in the world other than my wife.  Really fun, interesting, and caring.  At other times she is cold as ice - particularly at work (even though she is really outgoing with everyone else at work) ... to the extent that others have asked if she dislikes me for some reason.  Outside of work she invites me to parties, we sometimes get drinks together, and we talk all the time during our runs.  Although lately the 'cold' times have sometimes extended out of work.

I'm not really attracted to her and have no interest (my wife is amazing, and would never jeopardize us), but because of her hit/miss personality I can't help obsessing.  I've tried asking her about it once or twice, but she doesn't like to share feelings and just blowed it off.  

How can I figure out what's going on?  I've tried asking my wife, but she mostly thinks this is humorous and says she can't read people's minds either.

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: Friend is sometimes very friendly, and other times really cold.  I just want to be friends, but don't know how to make that happen.