The pasta water isn't salty enough
This is the most common reason. Pasta absorbs water as it cooks - if that water is bland, the pasta itself is bland, and no amount of sauce fixes it. The water should taste noticeably salty, like a light broth. Most people use a fraction of the salt required.
You're not finishing the sauce with pasta water
Pasta water is starchy, salty, and hot - it emulsifies fat and liquid in the sauce and makes it cling to the pasta rather than pooling at the bottom of the bowl. Most home cooks pour it away without a second thought.
You're pouring the sauce over instead of combining it
Plating pasta with sauce ladled over the top produces a dish where half the pasta has no sauce and the other half has too much. The pasta and sauce need to be combined in the pan - briefly - so every piece is coated.
The tomato sauce hasn't been reduced properly
Undercooked tomato sauce tastes sharp, thin, and raw. The natural sugars in tomatoes only develop properly with time and heat. Rushing this step produces a sauce that tastes like something's missing - because it is.
No fat at the end
Fat carries flavour and gives pasta sauce its richness and body. Most sauces benefit from a small addition of butter or good olive oil at the very end - off the heat, stirred through.