I think the meaning of life is you find your own meaning. I'm sure that's no great revelation, but there it is. I don't have the overarching goals of a deity hanging over my head, so the way I look at it, everyone is born, they have biological imperatives, emotional needs, genetic predispositions, learned preferences, familial obligations, societal expectations... Everyone, either individually or as a group, strives towards his or her own goals and what makes him or her happy. I refer to this motivator and these factors as the basis for social/societal evolution.
Largely, I believe in Maslow's hierarchy of needs. It is also known that the population growth rate in well developed countries is far less than that of less developed countries. I think these two things are linked by things like education and access to various entertainment. So as a person's baser needs are fulfilled and any obligations or expectations are met (love/belonging/esteem), he or she might turn to hobbies or self-betterment instead (being needs). In a well developed country, this would be far more likely, and so that's why I think places like America, Japan, and Europe would all see population declines if it weren't for immigration.
Abortion and assisted suicide are things that affect society and population (and the affected people, on a personal level, of course). On one hand, abortion does not give new life a choice in the matter, but in my view, new life isn't necessary. It isn't necessary in the sense that human civilization does not depend on the birth of another child to survive. With assisted suicide, clearly the person has a choice in the matter, but it is now society's (those that are opposed) intent to take that away. (Quite hypocritical, but that's for another time.) But again, human civilization is not dependent on the life of a single person, sci-fi movies aside.
To value human life is one of the basic tenets (if not the basic tenet) of morality (probably all of them, but I'm no expert). Specifically regarding assisted suicide, I believe it is more moral to aid someone in pain. Regarding abortion, I don't think I would ever, but that is my decision. Another person with a different set of circumstances (physical/emotional needs, societal expectations) may choose differently. It is my position that human life, per se, has no value; no more than any other form of life, anyway. It is what is done with that life, the ability of a person to fulfill his or her goals and ambitions that gives life value; that gives life meaning.
As society evolves, the ability to provide its members with the means to meet their higher needs, the needs of being, should be emphasized. It's ironic then that I think that two of the means might be for the cessation of being. The happiness a single person is probably not terribly important to society as a whole, but denying that person the ability to achieve happiness is a matter for all people. It is in my view, then, that it is morally imperative to allow individuals the freedom of choice governing their own life.