Why is it necessary to set your goals realistically?

Setting goals realistically, is not a requirement for one to exist.

One will always exist as their true self.

Setting goals realistically, will keep your investment of your energies within what you think is real. Once you achieve such a goal, you may temporarily feel a sense of success because you affirmed what you think is real by completing the goal. Yet, remaining within such limitations won’t be appealing forever, sooner or later one starts to feel constrained and wants to expand beyond it.

Ultimately though, all goals are possible, unless they are the very definition of impossibility themselves. It is just that, a goal might not be possible if you keep your self in a reality where you are preventing that goal from happening, that there is a contradiction with the definition of the goal and the reality within which it has been defined. In this case, either you let go of the reality you are keeping yourself in, or the goal fails, or you let go of the goal.

Nothing is necessary implicitly; all experiences are possible. But, some things are necessary within a certain context, within a certain reality to move towards the experience of more happiness, peace, creativity, intelligence.

For example, within this reality, if you keep investing your energies into shallow things and keep yourself unaware of who you are deep within yourself, and make this a habit of living your life to such a degree that it becomes your default understanding, then some types of activities are needed/become necessary to help you see outside this default understanding so that you can see the possibility of stopping investing your energies to the habitual understanding and starting investing to a reality where you experience the freedom, happiness, power that your heart desires you to consciously experience.

Even necessity is ultimately created by us along with the reality that we live in.

Goal is an experience, a direction that one defines within themselves so that they can use this to focus their energies towards what the goal represents within themselves. You can even set goals for this world that are not realistically achievable during your lifetime according to your understanding, yet a lifetime of work can be contributed to them so that they become closer to becoming a reality in this world.