Step 1: Treat one strand as a template. Each base on it can accept only one specific partner base on the opposite strand.
Step 2: Since A pairs with T and G pairs with C are the only allowed matches, the two strands can never carry the same order of bases. If strand 1 reads A-G-C-T, strand 2 is forced to read T-C-G-A.
Step 3: This strict one-to-one matching is exactly what complementary means. It also explains accurate replication, because each old strand can direct the building of a brand new partner strand.
Step 4: So the strands mirror each other as partners but hold chemically different base sequences. That is why they are called complementary rather than identical.