Purpose?

Although it has no doubt been questioned before, I would like to ask it once more: What is the purpose of life?

As everyone says, money can't buy everything and I won't for a second entertain such a notion. However, the money fueling such materialistic pretenses can allegedly not buy happiness, but is this to say that happiness is our purpose?

For instance, I can purchase an Xbox 360 and thoroughly enjoy myself and have times of great fun. However, once I progress from my Xbox stage and have had my fun, what comes next? In this sense, happiness is like a drug. As it has been described by drug addicts, the purpose for continuing to use a drug is to chase the first high or to continue smoking is to feel the sense of relieved stress that they felt the first time they smoked. If this is the case with happiness, then happiness can't be that good of a thing.

I have many a-time pondered upon this topic considering perhaps the purpose may be companionship. However, such a source of pleasure is not stable enough to be our sole purpose of "existence" (if we exist, that is). Our friends and family can provide times of joy, as well as distress, so at the troughs of these waves of emotion, the fickleness of friendship is questionable.

So if happiness - comparable to a drug - and friendship - unstable as can be - are not our reasons for living, what is the reason?