Its also easier to read and get better in the story when there is a style suiting your story (read bakuman xD)... Like shoujo's usually have twinklie girls eyes, hot guys, blush and like flowerly stuff with scenes. blablabla but psychological manga's are drawn more like eye-powerness >.O perspective stuff to make more impact, more realistically? . And you also have that the mangaka all have their OWN STYLE you can even go through mt's art gallery stuff everyone has its own style cuz you never copy everything completely. but it all just depends on the mangaka, i can't say there isn't a shoujo manga that will be like different style youknow.
Barajou no Kiss
Shounen Harems(usually with echhi lol):
Shounen Fight(epic effect to make fight more awesome):