"V'ish ki seitzei mimeno shichvas zerah veruochatz besoro bemayim v'tamei ad ha'erev.(15:16) - And a man who has a seinal emission should wash his flesh in water and he will be impure until the evening (erev)."
After someone falls, he must get out of the depressed mode. As long as he is not besimcha, he is guaranteed to fall into the Yetzer Harah's net again.
As the Pasuk says: Even after he will purify himself from his emission, HE IS STILL guaranteed to be TAMEI again UNTIL his outlook becomes SWEET (Erev = Arev = sweet).