I never saw a wild thing
sorry for itself.
A small bird will drop frozen dead from a bough
without ever having felt sorry for itself.
This is reminiscent of what Jesus said in his Sermon on the Mount: 
 Behold the fowls of the air: for they sow not, neither do they reap, nor gather into barns; yet your heavenly Father feedeth them. Are ye not much better than they? –  Matthew 6:26, KJV 
 If Lawrence’s imagery serves the same purpose of that of Jesus, this sentence asks the question: if a bird will live and die without self-pity, why should we humans, supposedly more intelligent and capable, feel sorry for ourselves? 
 The cold imagery of death invoked here ( dropped frozen dead from a bough ) is parallel to Lawrence’s own circumstances – he was a very ill person throughout his life, dying of tuberculosis at the age of 44. Yet he was very industrious and prolific in his work – he probably didn’t let self-pity get in the way of being an accomplished writer and academic. 
 
 Maybe having some amount of self-pity is unavoidable for humans. Having the so-called higher-order emotions comes at the price of envying the freedom the wild things have from these feelings that hold us back.
Self-Pity