photoactivated localization microscopy (PALM)

I have been reading up on PALM works and am still confused about a few things. In particular, (1) how can you only excite a few molecules at a time and then only photobleach those molecules? Won't other molecules also automatically be excited and eventually photobleached? (2) When you fit the point-spread function to a Gaussian function, what exactly are you doing? Isn't it basically just taking the weighted average of all the intensities at each pixel? Thanks!