There isnt really other answers to your question than the physical aspect of things and cuz evolution. Or there are some attempts in cognitive psychology to explain these things from a cognitive rather than neural perspective, but imo all of them are just more or less flawed descriptions about things that can only be explained from neural perspective. You might be interested in gestalt laws of grouping; . I dont remember the names for other theories alike, but maybe this is something to get you started with. Also i suggest searching google scholar for some articles and prepare to read walls of technical text, since many things regarding this topic cant really be explained in simple ways.