What does this mean “In love’s service only wounded soldiers can serve.”?

The mature people one meets often have this crooked timber view, having learned from experience the intransigence of imperfection and how to make a friend of every stupid stumble. As Thornton Wilder once put it, “In love’s service only wounded soldiers can serve.”

It means that you can only know something through loss, and I think bittersweet describes this maturation theme: Life and love have no perfection, and the illusion of the ideal mars what is the truth. The recognition that we are crooked timber. We are, to varying degrees, foolish, weak, and often just plain inexplicable — and always will be is in itself a prerequisite tool to experiencing life or love.

As Pat Benatar taught us, Love is a Battlefield.

This is a bit of an opinion, but the only way to truly appreciate love is to understand what it isn't and that is a processes that involves a lot of pain and hurt. So, by the time you get into love's service, you've already been wounded in the battle.

I don't personally care for the construction of the sentence, however. Having "service" and then "serve" feels clunky. I would probably change the last "serve" to "enlist" or something, but who am I to challenge Thornton Wilder (no one, that's who).