It’s not the alcohol, it’s the TASTE–which is a trigger (for beer drinkers, anyway) for happiness.
The taste of beer, without any effect from alcohol itself, can trigger dopamine release in the brain, which is associated with drinking and other drugs of abuse.
Using brain imaging, researchers tested 49 men with two scans, one in which they tasted beer and the second in which they tasted Gatorade. They were looking for evidence of increased levels of dopamine, the "happiness hormone." The scans showed significantly more dopamine activity following the taste of beer than the sports drink.