To live on the left is to live optimistically, believing in progress despite setbacks, hoping despite frequent disappointment, urging progress against rightwing nostalgia for illusory "better yesterdays". Life on the left means trusting that the better side of human nature can prevail against selfishness and greed. Good argument can always persuade enough people to see that a more socially just society is in everyone's best interests.Whether human nature can somehow prevail over such innate qualities as greed and selfishness is dubious and policies that rest on that assumption will be disastrous. It would be a cheap shot to say that Toynbee hasn't quite managed that feat herself, despite her belief that changing human nature on a societal scale is a reasonable objective. It is no surprise that anyone who gets human nature so wrong is likely to misinterpret what the likely effects will be of massive increases in welfare spending that often provide incentives for poor behaviour.
If you assume that greed and selfishness are inherent parts of human nature then you can begin to devise policies that direct those impulses to useful ends. If you make no allowances for such impulses then it is hardly surprising that 'setbacks' will constantly occur.
Whilst greed and selfishness aren't the most attractive traits, despite being universal, they aren't as destructive to a society as envy and resentment.