The desire for praise is the desire to obtain, as a sign that one is good, the favorable opinions of others.
Conclusion (it follows that)
One who aids other primarily out of a desire for praise does not deserve praise for that aid.
Evidence (because)
People merit praise only for those actions motivated by a desire to help others.
Evaluate
We're trying to prove that a certain group of people do not deserve praise for certain aid.
The evidence kindly provided us with a mechanism for proving that someone doesn't deserve aid:
action is not motivated → don't deserve by a desire to help others praise for it
So all we need to do to prove our conclusion is establish that are "not motivated by a desire to help others".
Goal
Look for an answer that gets us from
aiding primarily out → not motivated by of a desire for praise desire to help others