Mental energy is another way of using the 93% of our brains that we don't use. Where some people have the capacity to remember 15 consecutive digits, others have the capacity to move objects without touching them (though, rare). Or, to see the future. Some people can give energy to others better than others, and some take it away.

Two strangers meet on a street, and if they're both able to feed each other enough energy (whether it be through humor, compliments, or drugs), then they will have a joyous meeting.

The same as when someone drains the energy out of a room, just by entering.