A lack of courage, and doubt.
Its very easy to stay where you are now, in the safe and stable lifestyle you have. Taking the leap towards your dreams could disrupt that lifestyle and could even completely destroy it, and then comes the doubt, the self questioning and the negative thoughts.
What if it doesn't work out? What if i want to go back to the way things were? What am i giving up?
The more focus we give those thoughts and that doubt, the more we fear taking the leap towards our dreams.
The fear, lack of courage, and doubt feed off of each other, they amplify each other and make things seem much worse than they are.