my question is why do you express it? Why not?

Are you hoping to change someone's views:
Depending on discussion, maybe! Or give them food for thought. They may seem to brush off an idea but sometimes, in the back of their head the little mouse is spinning. i.e. I used to be very involved in the fight against puppy mills. Did a lot of sensitization, booths, etc. Show them what this is all about and how as a community, we can turn things around. It took years of frustration. I had to get away from it as it started affecting my life (I was "just" a volunteer) and others kept the fight and it paid off in Quebec. Lots of petstores now don't get their pets from mills, but come now from shelters. So yes, I will voice and do what I have to do when I know my fight is worth it. Even if only one person out of 500 walk the talk, it is a won battle. Not the war, but better for this major problem.

I always try to use the positive approach and would never want to take down someone who is not on my side. But I can be pushy and come up with very valid reasons to prove my point. If I am unsure, I will do my homework and come back later, if still needed.

Would I get into a conversation if not ask? It depends of the subject.

I ask because recently I've been subjected to others uninvited/unwelcome opinions and it got me thinking about why they felt the need to share their thoughts and opinions, what they hoped to gain from it. Why they believe their opinion has any relevance.