...but why? i think i should really read the "manga" (not doujins) or something, getting curious of the plot

Eating in secret...?

This image is related to this Japanese slang word / habit: "便所飯 (benjo-meshi, toilet meal)".

• Pixiv tag search
• Japanese Wikipedia: 'lunch mate syndrome'~'benjo-meshi'
• The New York Times: The Opinion Pages
• Blog entry(1): 'Japan’s lonely students eat in toilets!'
• Blog entry(2): 'Benjo-meshi'

Added lonely tag.