Firstly, a period or 'full stop' signals the end of a sentence, i.e., a complete statement or idea that is independent.
"I see. I see"
Two individual sentences. Two thoughts. That they are identical is merely incidental. Here it is either a simple repetition or, esp. in speech, depending on intonation, a very different pair of ideas conveyed.
"I see, I see"
One sentence. One thought. The second occurrence reinforces ("Have no doubt, I confirm."), modifies, (per intonation, cf. above) or restates differently to complete the expression.
The reasoning is more reflective in the second example.
"My house, my rules" vs. "My house. My rules"
I don't see the words as being extremely similar.
Smug is exhibiting or feeling great or offensive satisfaction with oneself or with one's situation; self-righteously complacent, where synonyms are self-satisfied, superior, complacent, conceited, self-righteous, holier-than-thou, priggish, self-opinionated. The stress is on the self-righteous attitude accompanying being pleased with oneself.
While I know that complacent means satisfied with how things are and not wanting to change them, I think the connotation is one of unawareness of actual dangers or deficiencies in oneself or something one is a member of, for example, a political party or a church
I think, for example, that much of the world has become complacent about AIDS. We just don't worry about it as we once did.
Condescending means showing or characterized by a patronizing or superior attitude toward others, showing that you believe you are more intelligent or better than other people.
Best Answer
It's getting on my nerves is the more idiomatic expression of the two. It means that something is very annoying and is driving me mad. The sound of that baby screaming is getting on my nerves.
It's my nerves could be used in many different situations, including to indicate that something was annoying. But it could be used in a lot of other contexts too. Watching someone climbing to a dangerous height, for example, might cause you to say I can't watch that any longer, it's my nerves. Or I have to see the doctor - it's my nerves, meaning that you had some sort of nervous problem.