To me, two-space will often break the flow of the text while reading, because the gap between the period and the next word often exceeds the gap between the cur

To me, two-space will often break the flow of the text while reading, because the gap between the period and the next word often exceeds the gap between the current line and the next line down. The gestalt theory of perception explains how we mentally group objects by their proximity. If the next word is closer to the bottom line than it is to the period that came before it, then the hierarchy of connection is broken. I haven't done a study, but it doesn't seem unreasonable to me that it would take slightly more cognitive load to rearrange that hierarchy in your head. I believe this may be why some people perceive two-space as "ugly". It also explains why text with a too-small line height is equally unpleasant.