I have trained in swimming for a big part of my youth, not competition but skills. When practicing swimming under water all teachers were certain to warn us to not push too far. If you are reaching your limits there is a small window where you don't feel anything anymore, which you can possibly ignore by a feeling of "I can do this", which makes you euphoric as well. Combined with lack of oxygen the euphoria will actually put your life at risk while your down there being happy...