All the inside jokes
Inside jokes are probably one of the biggest things that friends share between them and nobody else. You may be in the presence of two friends who keep laughing amongst themselves and you don’t know why or how it started. But an inside joke could be triggered by anything and at any time.
It can be a little annoying if they are constantly laughing and you are not part of the joke, you may be feeling left out. But do not take it so personally, because it happens all the time. Just get your own best friend and make inside jokes with them, don’t try to force your way into one.