Eating someone else's food

When we had done, [the waiter] brought me a pudding, and having set it before me, seemed to ruminate, and to become absent in his mind for some moments.
'How's the pie?' he said, rousing himself.
'It's a pudding', I made answer. 
'Pudding!' he exclaimed.  'Why, bless me, so it is!  What!' looking at it nearer.  'You don't mean to say it's a batter-pudding!'
'Yes, it is indeed.'
'Why, a batter-pudding,' he said, taking up a table-spoon, 'is my favourite pudding!  Ain't that lucky?  Come on, little 'un, and let's see who'll get most.'
The waiter certainly got most.   David Copperfield, Charles Dickens

Why is it so often the case that what someone else is eating is so much more enticing than what is on our own plate?   I have vivid memories of primary school lunches when I would have done just about anything to eat my friend's white sliced bread sandwiches and chocolate bar rather than my Mum's home-made soup, though nowadays I think I got the better deal.

And how much more difficult it must be if you have nothing to eat, but all around you is food, as is the case with the waiter in Charles Dickens's David Copperfield (1849-50).