Does logic only apply to certain human situations? For example, is it logical to fall in love?
Not sure if I like the way you use the word "logic" here, but I think I understand your question. Let me rephrase it for you:
"Do humans use reason or act rationally only in certain situations? For example, does a human use reason or act rationally when he/she falls in love?"
To be honest, it depends what "level" you are looking at. In many cases (such as falling in love) it does not seem that a person is using reason or acting rationally, but this can appear below the surface. To address your example, it has been shown biologically that the "main goal" of an organism is to keep its genes in the gene pool; that is, to reproduce. So though it may not seem like "falling in love" is a rational action, it is actually a rather effective strategy which the human body uses to increase its chances of eventual reproduction.
It is logical to fall in love with a person and not a painting, whatever feelings the latter might evoke. It would be illogical not to fall in love, given the opportunity, if the were reasons to pursue the relationship. So there is always logic involved, if only a little.
Consider the flipside, does love, or fear only apply to certain human situations? I would say not. There is equally always something involved in any situation to be reassured by or to be careful about.
Logic is based on the feeling of clarity, or what some have called consilience. It is just one of several emotional processes that are always active.
Falling in love is not a logical thing. It's the exact opposite: a pure emotional experience, possibly driven by the instinct to procreate and raise young, not as the result of a logical conclusion.
Think back to any experiences you may have had - when you notice that you aren't feeling right until that special person is with you, was it the result of a logical conclusion? No, it probably caught you by surprise, the first time it happened. You feel the attraction, rather than logically deduce it.
When you see that person and feel that flood of emotion, did you logically conclude that because they are X or they are Y, they are good? No, you just felt it.
True that you probably have some logical reasons for wanting to be around another person, but once it really kicks in, the driving force is pure emotion, not logic. It's like something else takes control of your emotions, and you're just along for the ride.
Also explains why romance gone wrong produces so many illogical reactions. When it works, it's indescribably wonderful. When it doesn't, it really stinks, for reasons that have nothing to do with a logical train of thought.