As you are probably aware, the definite article is the default usage when pointing out something that is unique in a restricted sphere. For instance, we say: "Her kite got caught in the top of a tree," even though the tree the top belongs to is indefinite.
In the sentence above, the phrase "during tough times" is what we would expect; however, saying "during the tough times" forces the listener to figure what is the restricted sphere of reference to allow use of the definite article. Since this phrase is actually fairly common, the listener immediately thinks of life as being divided into two parts: the tough times and the easy/good times. The speaker is then understood as referring to this tough part of life that all of us expect to face from time to time.