Why do people think I'm complaining when I comment on something?

But I'm really not? And I'll say things in a nice calm tone, and firstly I'm not even complaining just making an observation.

Like I was at a Baseball game with my brother in law. And I noticed all these people taking selfies with their Iphones and I mention to my brother in law: "Wow. All these people with smart phones!" and my brother in law was like, "it's fine…"

Or I was with some friends one time at a smoothie place. And as we were sitting, I noticed how crazy this machine was mixing stuff and I was like "Wow, that machine! That's crazy!" And then one of my friends was like "It's fine…"

I just don't get it! I was just pointing out something interesting, not complaining.

You are a bit too sensitive. Getting a response of "It's fine…" does not imply the other person thinks you are complaining.

BUT if you still think people feel you are complaining, why not prefix your comment with one of the following phrases:

Don't think I'm complaining, but…
OR
That…(whatever you're commenting on)… Is really interesting, …
OR
What do you think of (whatever) ?

To the last suggestion of "What do you think of…, " I'm betting that you'll still get the same answer, "It's fine." They don't think you're complaining, they just don't express themselves with much imagination.