Pessimism at it's finest. Have you been to the afterlife? No. Make no presumtions. What if your journey through the spirit world is harder because you didn't believe in one and thought death was the end?
I agree with the rule about doing whatever makes you happy, with a small note added to it, don't harm others.
i don't know if i would really call that pessimism.. it's not like letting it ruin my life or anything. it actually makes me want to live the best life i can, just because of the fact that it probably is/might be the only one i get. i'm really not making any presumptions about the afterlife, i'm just saying i know that i'm going to die. i don't know and will not know about any sort of afterlife until then, so i dunno, i'll just enjoy the time i know i have. also, i would definitely agree about not harming others.