You get so invested in an asset, a development challenge, an investment, and you lose objectivity to make rational decisions as you feel you have "put so much into it" that you that you put an unrealistic value on what you "put so much into".
Hands down... i mean, its hard to know when its time to reassess, to look elsewhere, to let the car go (in my analogy/real situation). But sometimes, just sometimes the answer is to let go.