From his character , maybe he was tired of being bothered by Marines , though after he joined , he got bored hence he said he was excited when Marines chased him again lol
from Oda's perspective , someone who can counter an Emperor's level of strength was needed ( because Oda designed the 7 Warlords and crews to counter 1 Emperor's crew ) , so why not Mihawk , he got the Emperors' lvl of strength , he wasn't a Revo , he wasn't an Emperor , he wasn't an Admiral , he was the WSS and Zoro's final opponent which wouldn't stop him from becoming a Warlord
from Oda's perspective , someone who can counter an Emperor's level of strength was needed ( because Oda designed the 7 Warlords and crews to counter 1 Emperor's crew ) , so why not Mihawk , he got the Emperors' lvl of strength , he wasn't a Revo , he wasn't an Emperor , he wasn't an Admiral , he was the WSS and Zoro's final opponent which wouldn't stop him from becoming a Warlord