The main reason why you should invest is because that decision will take some money away from your spending. And that by itself is the simplest method to be wealthier. A method that is mostly under you control. But consuming less could be a good thing, for a number of other reasons. For example, it will make you better able to live through rougher times: both in terms of training, and when it comes to available resources. It will also slacken your dependence on a monthly salary, which can only be stressful. But what is more interesting is that by investing you are better positioned to get out of the vicious circle of consuming -> working hard -> needing more of comfort consuming -> working with even more desperation ... and most people will certainly get the idea.
Investing entails full responsability for your decisions. It needs discipline to get an investment plan executed over the course of many years to come. For most people, this responsability is unpleasant. We can certainly help with this.
If you think however that this kind of commitment is too much for you, you can still take the approach of the masses: buying an apartament or a bigger one, having the bank making sure you pay your mortgage or else. This is still better than nothing.
At the other extreme, those who plan their investments with the only goal to cover the bare necessities have our full simpathy and we will always help them with a free acount here.
If the necesity of investing is still not grasped, you need to aknowledge the fact that most likely you ALREADY invest. Yes, your investments might be some poor deposits or debit cards. You should first solve this discrepance in your thinking: "I every day go to work to make money. Every day I ignore the fact that I probably don't make money on my earnings so far."
Ignoring is the root of much misery in people's life.