Basically the main character, Alan, marries his best friend, Denny, because they genuinely love each other, but mainly because Denny has been diagnosed with Alzheimers and he wants Alan to be able to inherit him him when he dies and to be responsible for his care when he gets into the late stages of the disease. The show is very crass sometimes and not at all politically correct, but the epic unashamed friendship between Alan and Denny is hands-down the best part of the show. The series finale ends with them getting married.

Watch this to get an idea of what the show was like: https://www.youtube.com/watch?v=4la8Izrwea8 (tw Alzheimer's)