Well, as explained by the Second Hokage :
Strong feelings in the Uchiha clan folks had the potential to release a certain type of chakra in their brains. This was visible in their eyes as a red color, which is the Sharingan. The stronger their emotions, the stronger their Sharingan powers grew. They were able to awaken Mangekyo in cases of extreme mental trauma [either self-inflicted / suffered]. Very few have actually done it in the series.
Having said that, Kakashi did not awaken a Sharingan, it is Uchiha Obito's eye.
He was able to use it by means of his own chakra, but since that did not originate in him, the amount of chakra consumed was higher. Similar to using a scroll for a Suiton [Water] style jutsu, without having an affinity for that type.
Since, he could not control the sharingan to the level of actually using whenever required, he covered it with his headband. This doesnot mean he was using the sharingan always, he had to lift up his headband to use it [during combat / when required]
He does have vast reserves of chakra as displayed by his use of copied techniques, body activation and his own elemental manipulation, which are not easy without that. But Sharingan is something external to him, and he needs to focus more chakra to it.
Wanted to add upon the wiki reference with some additional info.