Synonyms for Why:

n.

keep, justification, necessity, start, call, kindly, cause, occasion, where has someone/something gotten to?, what's gotten into someone?, just, reason, what has/will become of, do with, anyway, what (has) happened to something, how, ground. do you mind?, who asked you?, proof, argument, suit yourself, get lost. end, target, planned, objective, intent, mark, design, intention, purpose, point, view, meaning, object, aim, ambition, goal. •reason (noun)
how, proof, cause.
•why (noun)
wherefore, how come.

Other synonyms:

intention, goal. intent. necessity, ambition, target, purpose, meaning. proof, justification. ground, object, argument. objective, cause. reason. view. occasion. end. point. mark. •Other relevant words:
intent, wherefore, design, mark, how, argument, call, end, necessity, object, intention, purpose, target, justification, view, ground, point, aim, proof, objective, ambition, meaning, reason, occasion, cause, goal.

Usage examples for why

1. Why does he not go? – Jan and Her Job by L. Allen Harker

2. Why do you do this, Mr. Siward? – The Fighting Chance by Robert W. Chambers

3. Why do it, then? – The British Barbarians by Grant Allen