When we talk about the purpose for our lives, what are we thinking about? It seems to me that there are five conceptions of what the purpose is.
1) A personal goal. That is, a purpose the person selects, because it satisfies a desire of his own. I decide I want to be a fourth-grade teacher, so I start taking classes at Glendale Community College to fulfill that goal.
2) An inherent purpose. Somehow inherent in the object itself is a purpose for existence. If we say the purpose of your eye is to see, perhaps what that means it that the ability to see is part of the nature of eyeness, as it were.
3) A satisfying purpose. It wouldn't make sense to say a rock has a satisfying purpose, but a person is satisfied if it achieves this, and we can also speak of a pig satisfied as well. "You achieved your goal of becoming a millionaire, but you seem dissatisfied nonetheless."
4) A given purpose. Someone who is responsible for our existence has a goal for our existence. This however, might not at all be a purpose that we would select for ourselves. For example, the purpose for which we raise beef cows it for us to eat their meat, which requires us to kill them in order to achieve that purpose. But of course the cows don't want to be killed. On some Calvinistic theological theories, the reason that some people exist is for them to sin and for God to eternally demonstrate his wrath towards them. However, even though those damned to hell achieve the purpose God has for them, they receive no satisfaction from that. One possible thing we might mean when we say that God loves someone, is that we think that the given purpose for their existence, the one God wants for them, is also an inherent purpose and a satisfying purpose.
5) A Darwinian function. A pure Darwinist, one who excludes all intelligent design from the explanation of anything in nature, can say that the purpose of the eye is to see, but what the Darwinian means by that is that the structure of the eye was selected for by the ultimately blind process of natural selection because it made seeing possible, and seeing was helpful in enabling the creature to survive.