Laughter, it is no surprise, is very contagious. When we hear laughter we often take part, or at the very least, crack a smile. When we get really caught up in laughter the cycle continues in an open loop circuit, perpetuating itself until we are unable to breath or are in tears.
Consider the events of 1962 where an outbreak of laughter in Tanganyika Tanzania. It started at a small boarding school where twelve to eighteen year old school girls started a six month long bout of laughter. The laughter was so severed the school had to be temporarily closed, but upon its closure, the children carried the infectious laughter to other parts where it spread to other communities. While certain logistics makes it impossible to endure such a long bout of laughter, particularly because it makes eating and sleeping impossible, let alone the acuteness of loss of breath and dizziness which would occur, it still helps illustrates the infectiousness of the laugh that we have all experienced.
The addition of laugh tracks to television situational comedies is another artifact of our desire to join in with laughter. If you have ever watched a similar comedy absent of the track or a studio audience, you will have noticed that it becomes obvious how important laughter is in the jokes effectiveness. Instinctively during a conversation or joke, we all know exactly the right time to laugh and even laugh, absent of anything else, still elicits laughter, showing just how pervasive it is.