"It's my family and I'll be going elsewhere. It's best I don't talk about it too much," he said, before welling with tears.
British newspapers believe Hamilton has fallen out badly with his former mentor Ron Dennis over his Mercedes switch, but he said telling boss Martin Whitmarsh about his move was "the most difficult (phone) call I've ever had to make".
"We'd grown so close and he had been so supportive and I didn't want to let people down," said the British driver.
"But at the end of the day you have to let people down sometimes to make decisions."
Hamilton likens leaving McLaren to finally "leaving home", and so Whitmarsh admitted he hopes the 27-year-old returns.
"We'd have him back, yes," he told British Sky television. "So let's hope it's just a gap year."