Answer:
Having too big of a vision in life can lead to disillusion and unhappiness sometimes, because if your dream is too big of a dream for you to handle, your mind starts to realize that fact as well as you, and day after day, you realize you may never complete that goal. That would lead to unhappiness due to the realization of this dream either just being that dream you had when you were younger but was impossible, or a dream you knew deep down inside you couldn't do(but still tried to). And with that, it would lead to disillusionment because you finally are accepting the fact that the vision of a perfect life or goal you dreamed of will never come true.
Hope this helps, and I hope you learned something new. Have a great day!