Why does Lewis structure for O3 have 1 double bond & 1 single bond instead of 2 double bonds?

Think of it this way, O3 is a neutral molecule (no charge), so if you have O=O=O, you would have a 2+ on the middle oxygen, which means 2+ charge on the overall molecule. Of course, we know this is not true, so using the valence bond model, the best representation is the O=O-O where we will have a 1+ charge on the middle oxygen and 1- on the oxygen to the right (total charge is 0). this structures is favored since it stabilized by resonance.