Many flowers are optimized for being easily found by insects, who don't have particularly good eyesight. To stick out from their surroundings, they can use bright unnatural colors (i.e. not green or brown), unusual patterns (concentric circles is a popular one), have a large surface, etc.

Also, flowers are often quite short-lived, and thus mostly undamaged; we find smoothness and symmetry attractive (for evolutionary reasons - they're signs of health in a human).

In addition, humans select flowers that they themselves find pretty to place in gardens and the like, so when you think of "flowers", the pretty varieties are more likely to come to mind than the less attractive ones (like say that of the plane tree, or of many species of grass - many flowers are also prettier if you look at them in the UltraViolet.). If you take a walk in the woods, most plants you encounter won't have flowers you'll find that pretty; ugly or unremarkable flowers may not even register in your mind as "flowers".