Happiness is:

[evan]

snipers in a night assault; especially when the 25 pounders open up on where he was two turns ago.

[desh44]

Airdropping a string of scouts and snipers behind the line and having them take out a couple of batteries of field guns

[Marek_Tucan]

Happiness is your motor scouts with light tank support finding an unguarded spot and penetrating to enemy gun and mortar batteries.

[Artur]

When your opponent is cooperative to your battle plan .
(Sometimes this also must be very suspicious...)

Artur.