Since a sequence of single-stranded DNA needs to find its complementary strand to reform a double helix, common sequences renature more rapidly than rare sequences. Indeed, the rate at which a sequence will reassociate is proportional to the number of copies of that sequence in the DNA sample. A sample with a highly-repetitive sequence will renature rapidly, while complex sequences will renature slowly.  
However, instead of simply measuring the percentage of double-stranded DNA versus time, the amount of renaturation is measured relative to a Ct value. The Ct value is the product of C (the initial concentration of DNA), t (time in seconds), and a constant that depends on the concentration of cations in the buffer. Repetitive DNA will renature at low Ct values, while complex and unique DNA sequences will renature at high Ct values. The fast renaturation of the repetitive DNA is because of the availability of numerous complementary sequences.