It is some deep psychological issues with me about that.
Firstly, I don't want to stick out from the crowd and be the only one who gives a laugh reaction, so I don't hurt someones feelings. So I wait until someone else have given it a laugh reaction.
Secondly I feel like it would be strange if I come in and laugh at something posted a couple of days ago if I was not part of the chat when it happened.
These personal issues are not always present, but apparently present enough for you to notice that I am "strange" in that regard.