If I had lived in the Imperium, I am sure my greatest wish and secret fantasy would be to become a Space Marine.
The Space Marines, their role and work, cannot be taken out of context, so you really have to answer this thread's question in light of the culture of the 40K universe. If you are a loyal Imperial citizen, this means that the whole point of your existence is to serve and to die for the Emperor. Sacrifice, both personal and in a broader sense, is not so much a choice as a neccessity in the Imperium. That only one in every ten thousand or so survive the recruitment process is not a reason to not want to be a Space Marine. Neither is the forfeitment of sex and human contact, the horrors you are likely to experience or the hard and monotonous life. If your are a Space Marine, then you have reached the pinnacle of what a human can become, both in purpose and status. You will live and die a hero, a saint, and will be remembered for millennia after you have left this cruel galaxy to join your father, the Emperor.
However, as has been pointed out before, becoming a Space Marine is not a choice. It is a duty.