Why do you need to express yourself? I'm not sure in what context, how to interpret your question. If you are saying you do have a need to express yourself but asking why, my answer is this. It's that we as a social species have a need to be included, accepted and above all, understood. This would be the reason one would be compelled to express themselves verbally to others and is natural.

If you are asking why it should be necessary to do so, my answer would be that it isn't entirely necessary to function in groups or society as a whole but it does a few things, related to what I've said above. If you had no need for acceptance, and companions or friends, this would not be a subject to even breach. Even those that are most introverted such as myself, have a need to be part of the whole as an accepted part of society, and although I do alright staying mostly as a loner, I do have the need to interact and attempt to have friends or acquaintances at the very least. I won't claim that I do much of the things I say very much although I understand that my reluctance to interact more than I do is entirely on my shoulders. At this point for me, I know that this is my choice to remain mostly alone.

Of course it's completely understandable to feel stupid for your opinion. There are plenty of people out there that would go out of their way to make sure others that disagree with their opinions are "wrong" and "stupid" for seeing things differently. I correct people all the time on forums for stating their opinions as fact and for belittling people for their differing views because it's a subject that truly is one I feel very strongly about.

"I should just stick to facts..." I disagree, although I can understand why you would say that. a conversation limited completely to factual information isn't one that typically needs to be spoken anyway. Talking about how some subject factually is dry and unnecessary. Except for if one is explaining facts to someone that may not understand something those conversations are not the ones that bring us together, help us get to know one another or motivate us to become friends. Our views on those facts are what make us individuals, make us interesting and that is how we get to know another person beyond the superficial.

We can, you and I talk about what, for example about the subject of religion. Factually we can have a conversation about what religion is but what would make the conversation one where we got to know one another would be talking about what your view on it and mine was, whether we agree or differ. That is the soul of good conversation not the facts.

I've rambled enough about conversation itself.

I feel, and I may be wrong but I will state this anyway, that it is your confidence, your belief in yourself, your views and your standing in life, career and as a peer where you need to be strengthened. Stand strong in what you believe, your opinions and never make apologies for them, that makes you who you are, and no one has any right to tell you that's wrong. Once you take that to heart and understand it, those conversations with people will be much easier to have.