Essentially thunderstorms are the result of an unstable atmosphere which is caused by warm air underneath much colder air.
As a result, when the warm air rises it cools and condenses forming small droplets of water.
Thunderstorms thrive under certain conditions, namely when the two basic elements are in effect: moisture and rapidly rising warm air.
These storms are more common during spring and summer because moisture and warmth are crucial parts of creating thunderstorms.
In particular, thunderstorms are common in humid areas.
High humidity areas, in conjunction with warm temperatures, create massive amounts of warm, moist air rising into the atmosphere where it can easily form a thunderstorm.