Actually, you can. It is rather simple. You can influence other peoples opinions with sound arguments and supporting evidence. It is a tried and tested way of advancing ideas and thoughts, and has a much, much, better success record than simply stating postulates and telling people to figure it out themselves.
But you are quite correct. People are allowed to have opinions. In a discussion forum, such as this one is, people are encouraged to have opinions. However, opinions in and of themselves are rather useless for a discussion. In a discussion forum, people not only state their opinions, but also...well, discuss them. If all you can do is state an opinion, and refuse to do anything else, well, you aren't really discussing anything, are you? At which point, everyone else has the right to come to the opinion that your opinion is of little relevance to the discussion, since you are so personally indifferent to supporting it in any meaningful manner.
We refer to this as intellectual laziness. If you do not have the urge to support your opinion, don't expect anyone else to. And don't be surprised when your opinion isn't given any weight.