Some things are facts backed up by hard evidence. Humans need Oxygen to live, that's not my opinion. It's important to note when you are stating an opinion. Not least because it can help set the right tone for a discussion.
If you state "this is my opinion, what's yours? I'm open to changing it if I hear a good argument or am exposed to new evidence" that sets the tone for a much different conversation than "these are the facts in my universe, for all you know they may be beliefs for which I would rather die than acknowledge they are even the slightest bit imperfect". When you say "I think ..." or "in my opinion ..." you are signaling something closer to the first message, when you don't you may be signaling something closer to the 2nd message.
If you state "this is my opinion, what's yours? I'm open to changing it if I hear a good argument or am exposed to new evidence" that sets the tone for a much different conversation than "these are the facts in my universe, for all you know they may be beliefs for which I would rather die than acknowledge they are even the slightest bit imperfect". When you say "I think ..." or "in my opinion ..." you are signaling something closer to the first message, when you don't you may be signaling something closer to the 2nd message.