'past love' - think of it as another way of saying 'ex-girlfriend'. In this text, we don't care who the ex-girlfriend or 'past love' was so it's just A past love, not a certain one, so no use for a DEFINITE ARTICLE (the). The use of 'too' here is required to explain what the 'he' might have been thinking about. The writer is saying that maybe the man HAD been thinking about an ex-girlfriend' but NOT too much. Info: 'too' is similar in meaning to 'excessively'. HtH - Rob.

"It was unlikely that he had been spending too much time thinking about a past love or that night's dinner." 1. TOO MUCH TIME means more time than necessary. MUCH TIME means a lot of time. I don't know the context of your sentence. But if I spend TOO MUCH TIME thinking about the answer to your question, then it means that I spent more time than necessary. But if I spend MUCH TIME thinking about the answer to your question, then it means that I took a lot of time to give you a thoughtful answer (but not necessarily too much time). 2. Thinking about A PAST LOVE means thinking about a somebody in the past that he was in love with. The word love refers to a person. Thinking about THE PAST LOVE makes no sense without naming a person. "Julia was THE PAST LOVE that he spent so much time thinking about, not Andrea." I hope this helps.