This first post is about how simple yet subtle style can be. Smart dressing can sound so easy: dark suit, pale shirt, dark tie. Then you see someone with a striped shirt under a striped suit. The stripes on each are the same width apart; suddenly it looks like he's wearing pyjamas. The items on their own were fine, luxurious even. But they don't work together. He might not know why, or care. It's a subtle problem, solved by a simple guideline.
That guideline is that patterns have to balance. Don't wear a striped suit, shirt or tie next to each other unless they are sufficiently different. A dense, pale, striped shirt may be fine against a sparse, dark, bold pinstripe. It can get more complicated than that, but this is a simple starting point.
Suits look good because the plunging V of the chest gives an impression of being slim and broad. This is amplified by the fact that in a normal, lounge suit the jacket and trousers are the same colour - it creates a long, lean silhouette. If the shirt and suit are the same pattern that effect is lessened. You are a single block of pattern and may as well be wearing a jump suit.