Why can't I take credit for someone getting saved?
You cannot take credit for anyone's salvation because you did not save them—God did. Whether you plant or water, your role is only to sow the seed; God gives the increase and finishes the work of salvation. Paul himself refused to take credit, since salvation is God's work.
Les explains that whether a believer is the planter or the waterer, or does whatever other part of the work, that isn't what counts. Neither the one who plants nor the one who waters is anything; God gives the increase. Paul does not even take credit for it himself. A person cannot take credit for someone's salvation when someone else has had a part, even if he was the only one, because he did not save them—God did. The role of every believer is to sow the seed, sometimes with just a couple or three words or a little statement that makes someone sit up and take notice about eternity. Then God moves in and finishes that work of salvation.
Now he that planteth and he that watereth are one: and every man shall receive his own reward according to his own labour.