It's a quote from the author T.S. Eliot. In this quote, "the want of a motive" is actually a bad thing. It uses the verb "want" differently from how it is often used in English.

"Want" typically means to desire something, but T.S. Eliot is using it poetically in this case. In this case, "want" means "lack of". Sometimes "want" is used this way because we want the things we lack.

So the phrase "want of a motive" actually means "lack of a motive". What he is trying to say by "What makes life dreary is want of a motive" is that life is boring and pointless if you lack a motive or desire to do something.

So yes, in this case, "the want of a motive" would be good and desirable to have.

I hope this helps!