Why does it grow in Summer only?

Not only watermelons but this season we get grapes, mangoes etc. -- all water based.

The prosperity of the world depends not on our own efforts but on the background arrangement of the Supreme Lord, directly carried out by the demigods. -- BG 3.16 purport by Srila Prabhupada

Just imagine, someone in the background is always thinking about us and making meaningful arrangements. If we just see this -- see the background in every situation and not just get sucked up totally by the foreground then we can be so happy.

HARE KRISHNA!!!