Actually, suki desu doesn't mean "I love you", but "I like you". "I love you" would be "aishiteru", and it is seldom used by "normal" Japanese people.
The word "suki" by itself means to like. But Japanese is a highly contextual language. In a situation where one person is confessing, "suki desu" is better translated to "I love you." If you're pointing at apples and saying "suki desu" then you're saying I like apples.