Why are thunder storms usually accompanied by lighting and thunder?

They ALWAYS happen together.

The lightning you see is an electrical discharge from ice crystals rubing together in clouds. This makes an almighty crack sound, and a bright light is observed. You hear the thunder after you see the lightning because sounds travels slower than light. But in fact, the sound and the light came to be at the same time.

The further away you are from the strom the larger the delay between seeing the light and hearing the sound is.