Saying “I love you” isn’t that common to say in Japan as much as saying “I love you” in western countries. But of cause sometimes we have a chance to say that. Only sometimes. Because saying “I love you” in Japanese is not that casual like in English.
People in Tokyo would say “Aisiteru” or more politely “Aisite imasu”. But we would become a bit too shy to say those phrases. So often we say “Sukidesu” or “Anatano kotoga sukidesu” which means like “I like you”.
On the other hand, people in Kansai area (incl. Osaka) can say it more casually like “sukiyanen” or “meccha sukiyade”. This is kind of less humiliating to say because they sound a little more casual.