The purpose of life is to cook for you, clean for you, and satisfy your every desire for the rest of your wife. It would also help if she could point out any unfortunate spoonerisms you make at inappropriate moments.
On a more serious note, I think the question, and so the answer, can be split into:
- the purpose of a life;
- the purpose of life in general;
- personal purpose,
the difference between the first and last being that the purpose of a life holds for all life forms, while personal purpose is unique to each.
For the purpose of a life, we can look to what defines it. A living thing metabolises, grows and reproduces. All else is accidental, so you might say the purpose of a life is to metabolise, grow and reproduce.
All that is rather specific to one life. The metabolism of some life form long since dead is hardly relevant to you or me today. Reproduction is relevant, for were it not for my dear mother's ill-thought-out night of sexual abandon, I would not exist and would not have a life to have a purpose. The impact on my life of the reproduction of my ancestors is the realm of genetics and evolution. I'd say the purpose from this viewpoint is one of natural selection: to preserve the bloodline and refine the fitness of generations to come by advantageous sexual selection. I am proud to be a blight on the purpose of life in general, then. \o/
And finally personal purpose. This is not something intrinsic like the previous examples, for the purpose itself is unique, must be determined and may be transient. This is the realm of existentialism. The purpose of life is the great existential problem we must all face. Some people opt out of the problem by embracing some pre-packaged solution, such as religion, capitalisim or heroin addiction, while others permanently defer the problem, say by spending their lives drinking lager in front of the television. Others find something to dedicate their lives to and maybe achieve great things in one or more field, maybe achieve nothing despite trying. I'm not sure it really matters what approach you take, if any. There is no wrong or right path to existential relief, but I think most people would agree that dedicating your life to helping homeless kids is probably more rewarding than dedicating it to relieving yourself of junk sickness. And people will think you're a lot cooler, so you'll probably get a better range of prospective mates to refine your progeny's fitness for survival. The best way, of course, to find a quest you will be happy to dedicate your life to is to broaden the scope of your experience. Some people do this with travel, others work, others means of expression, others sex, others drugs... Plenty of areas to experiment in.
So I'd go for:
1. follow a healthy, balanced diet;
2. follow a good exercise regime;
3. choose your sexual partner(s) well;
4. shag often;
5. experience as much as you can, preferably without debasing yourself;
6. find something you like and hold onto it with both hands;
7. when you're done growing, child-rearing and achieving, do all the things you didn't let yourself do earlier in life just in case.
It's not a bad philosophy. Look after yourself, broaden your experiences, embrace free love but be discriminating, try to achieve something in your life, then screw it all and become a reclusive crack addict!