Reptiles
Dinosaurs are more closely related to mammals. Both mammals and dinosaurs share a reptilian ancestor that they don't share with amphibians.
Amphibians and avians are the most closely related species to reptiles.
Yes, they are close related, but reptiles tend to have scales and amphibians tend to have smooth skin (mostly on tropical amphibians) and even slimy skin.
Tetrapods
Dinosaurs are more closely related to reptiles than to amphibians. In fact, dinosaurs are classified as reptiles. Amphibians are lower on the evolutionary tree than reptiles. Reptiles developed from amphibians. Amphibians developed from fish.
Salamanders are related to newts.
Neither...They were reptiles. dinosaurs were nether amphibians or mammals they were all reptiles, though they are more closely related to birds.
Animals
Salamanders are amphibians, related to newts and frogs. Despite the similarity in appearamce salamanders are not closely related to lizards.
Family and kingdom
The group of fishes most closely related to ancestral amphibians are the lobe-finned fishes, which include the coelacanths and lungfishes. These fishes share certain characteristics with amphibians, such as limbs with digits and lungs or lung-like structures, indicating a close evolutionary relationship.
Convergent Evolution is the reason. :)