Why you cannot afford desparing?

In John 6, after eating a free meal a crowd of people wanted to get more free meals. Jesus refused to entertain their request. Rather he started talking about serving them with another food/drink, that is, eating Jesus' body and drinking his blood. People despaired. They gave up following Jesus. Many 'disciples' also stopped (or at least started considering to stop) following Jesus.
 
Despair means:

1. To be hopeless; to have no hope; to give up all hope or expectation;
2. To give up as beyond hope or expectation;
3. Loss of hope; utter hopelessness; complete despondency.

People despair for a number of reasons. In case of the crowds of people in John 6, they despaired for they could not get what they wanted - more food. 

Some despair with themselves for they fall into the same sin problem again and again. They could not get more out of themselves so to speak. So they say, "I am hopeless."

Whatever the reasons, despair is not good. 

Thus a few brainy quotes on despair:
  

Despair is the conclusion of fools. (Benjamin Disraeli)

Only a man who has felt ultimate despair is capable of feeling ultimate bliss. (Alexandre Dumas)   

Action is the antidote to despair. (Joan Baez)  

There was a castle called Doubting Castle, the owner whereof was Giant Despair. (John Bunyan)

Still you might feel like succumbing to despair. If indeed you want to give up for good, listen to Jesus' final argument against your decision not to believe/follow any more: 

"Aware that his disciples were grumbling about this, Jesus said to them, "Does this offend you? What if you see the Son of Man ascend to where he was before?""(John 6:61-62)

This argument speaks of the way in which the path of life is built: that is, it leads upward for the prudent to keep them from going down to the place of the dead. Pro 15:24

The point of Jesus' argument is that if you can afford going down to the grave, go ahead. Otherwise, keep believing and following Jesus.