They have told you the reason. You may be sure that their reasoning is wrong, but the granting of visas is not a "do all the right things you get one" situation. They want to see that you have reasons to return to your country, and that you won't come for a visit and just decide to stay.
Examples of strong ties are owning property or a non-portable business, having a family such as a spouse or children, having a job that might fire you if you just didn't come back from Norway. You are young, unencumbered, self employed, and have enough money for a holiday, but not so much that it's impossible to imagine you leaving your current life behind for a new one in Norway.
As time goes by your ties to your home country will strengthen, and perhaps a visa will be granted later. There are lot of questions on this site related to the topic. You can't fake it up by quickly buying something or whatever. And you can't say some magic words in a letter that will make them believe you are not a risk to stay. Other people lie, and you are judged because of that.