Moments are pearls which becomes valuable by age and store in our subconscious mind. The older we grew up we start remembering those moments and start laughing. Hahahahahahaha....
There are many things in life which are important and add a flavor in remembering the moments which were covered in life's span, we usually laugh by bringing back some of the interesting facts from our sub conscious mind towards our conscious one. These are called boom up factors, which really make our life so pleasant that we usually discussed in our conversations, the best thing is we are not bored by repeating those moments which blinks our facial expression and I must say it is deeply rooted in our life's cycle.