Relationships with people are hard to understand, but it seems that building them or establishing contacts is easier for some people than it is for others.

I thought about it recently and I came up with the following.

It is easy to draw a line between two categories of people. The outgoing ones and the reserved ones. I agree things are not black or white, but it is simpler to explain my point.

The difference between the two categories comes from a simple idea, I think. The outgoing ones have a positive affect for everyone by default. They like everyone if you prefer. They then wait until they know the person more to dislike him. I think this is the origin of their ability to chat and to be friendly with everyone.
On the contrary, reserved people have a rather negative or at least neutral affect for people by default. They then wait to know the person to change their affect from neutral to positive. It is therefore harder for them to be outgoing. They will be friendly after a while once their affect has changed whereas the outgoing ones will be friendly at the very beginning.

I agree it maybe answers the questions too simplistically, because other factors such as confidence may have an influence too. However, it points out a rather big difference which is far from being senseless, in my opinion.