The purpose of life seems "to be". All we know for sure is that we are here and alive (however we as unique individuals may perceive that). We can know for sure nothing more than that we are alive and in the process of being and becoming. The best thing we are sure we can do in life is to be, because that's a big reason in and of itself to be alive! We've little to lose by being alive, but a lot to lose by being dead (eg, missing out on opportunities that we may be unable to appreciate right now or because the current limits of reason prevent us from appreciating them).
Furthermore, it would be a tremendous loss to destroy our existence because if there is a heretofore unknown "purpose" to our existence, we wouldn't be able to come back and experience that existence. So why not, then, stay alive and be—we won't really lose anything by doing this, but we would lose a lot by ending our lives. Besides, for most people life is too fun or enjoyable to die—especially when they feel accepted, loved, and good about themselves. This is, of course, based on a somewhat rational/economic view of life, both of whose assumptions are themselves open to debate.
So some more ideas on therapy... Therapy is there to help us be. However, why should therapy help us with being and not with suicide? Notably, because people who want suicide will not come to therapy in the first place. For those people who aren't sure whether they want to live or die, therapy can help them make the best decision for them. This is not, however, utterly dangerous (from a moral perspective), because in the natural experience of therapy, clients will (hopefully) realise what I've just written in the previous paragraph. But in essence, therapy is about therapist and client embarking together on a process of being. Wherever this process will lead—even to new philosophies of life (or death)—is good and therapeutic.
On a final note, I want to make a mention of happiness and muse a bit on language. Mark Kingwell, in his book Better Living: In Pursuit of Happiness from Plato to Prozac, writes that happiness, for him, is to feel that you are living a life worth living—and this includes the life that consists of "just being".
When we talk about happiness, we often talk about the "meaning" of life. In doing so, we use language. But language has some problems in this regard. In explaining ontology (the basis of being), language becomes a tautology. Language doesn't give us any new, illuminating, truly objective information about life and happiness because it is a part of life itself. Something can't describe itself objectively. And it seems that there is no other system on earth that will provide us with any objective illumination. Furthermore, we are attempting to use linear systems (language being one such system) to describe what is most likely a dynamic thing.
(See also my 30 October
2002 entry on why more people aren't killing themselves).