Why to people share?

There are not many people who work with me, but some how I have managed to never work with on particular person. He gave me my application and I have seen him 2 other times since I have been working there. Yesterday was the third time of seeing him, and it was for a total of five minutes. In that time I had one of the most pointless co-worker conversations; and I have had a lot of pointless conversations at my jobs over the years. He came down to where I was working so that I could go to a different department. While I was finishing up what I was doing, the following conversation took place:

"I know that we haven't known each other very long, (do we know each other now?) but I wanted to let you know that my birthday was on Friday."
(I assumed this was where he would say 'and I am having a gathering if you want to come') Silence......what was it that he was looking for me to say?
I replied, "Oh really? How old will you be?"
End conversation.

What was the point of that conversation? Had we been talking about weekend plans then sure, it is appropriate to say 'it is my bday!' We hadn't even exchanged the typical "how are you/how was your day" yet. What motivated him to tell me it was his birthday? Since we 'don't know each other very well' then what is the reason I needed to know it was his birthday? Maybe it is because I don't tell people random things about me that people I work with always think I am quiet at first......