Today's joke is based on two meanings of bring down.
Bring down: Make someone depressed, unhappy or exhausted. Get Down can also be used:
"This rainy weather is really bringing me down."
You can either go 'up' or 'down' in an elevator. The second part of the joke is based on the word uplifting - making someone feel better:
"The singer gave a very uplifting experience."
Note: Elevator is used in American English; Lift is used in British English.