Why do we add "the" before timings such as "morning," "evening," and "afternoon," but do not do so before timings such as "lunchtime" or "dinnertime?" For example, in "I will see you at lunchtime" as opposed to "I will see you in the morning"?

In English, it's not always easy or logical to state a rule for the use of articles (a, an, the). Here are a couple of webpages that try:

http://grammar.ccc.commnet.edu/grammar/determiners/determiners.htm
From http://grammar.ccc.commnet.edu/grammar/index2.htm

http://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm
From http://www.englishclub.com/