Shiva is said to have consumed poison which formed a lot of heat. Water is dropped to reduce the pain caused to Shiva due to his heat
Another cultural belief says that Lord Shiva is Abhisheka Priya. This means he likes it when his devotees offer him water or milk
The water falling over Lingam symbolizes the River Ganga flowing through his hair
According to Science, Lingam is a structure of nuclear reactors, and the constant dripping of milk or water on Shiva Linga keeps the structure cool.
Another research shows that Shiva Linga symbolizes the black hole, and the constant water dripping on it shows how Shiva Linga is consuming energy
Shiva is an ascetic god and spends most of his time meditating. Hence water is continuously dropped over his head to keep him engaged with worldly matters.
There is a third eye on his forehead, which he opens when he is enraged. To prevent this, water or milk is dropped continuously over Shiva Linga to keep him cool.
Shiva Linga emits powerful energy; however, not everyone is spiritually inclined enough to absorb these rays. Therefore, water is dropped over Shiva Linga to remove these rays
Pouring holy water on Shiva Linga releases powerful vibrations and energizes the deity.