Santiago first realizes that he has developed a tendency to speak his thoughts aloud after "the boy" (meaning, Manolin) left. After that, he has not had anybody else to talk to and, as he is poor, he deems it a normal behavior to speak to himself since the rich have the luxury of having a radio to speak back to them and not let them feel alone at sea. As an old man, he tells himself (in aims of explaining his own behavior), that there is nobody to annoy now, so why not just speak to himself.
The man had "sung" alone before when he would fish at night but, as the narrative explains,
He did not remember when he had first started to talk aloud when he was by himself... He had probably started to talk aloud, when alone, when the boy had left. But he did not remember.
The old man kept with the tradition of never speaking "unnecessarily" while at sea. This is a commonly-known practice to any fisherman; it distracts the fisherman and detracts from the much needed concentration in terms of sensing where the fish may be. Yet, although the old man respects such tradition and abides by it, it is clear that he is in need of much needed company, that the fish and nature have become his new friends and that, more than likely, deep inside his heart, he misses Manolin.