Because jokingly saying "Why would you pick Maokai into Leona top? ;D" as a joke because nobody could know you are going Leona top, is not Toxic. And if you never expect compliments, jokes or mild trash talk from the enemy team, disable /all chat, it literally serves no other purpose.
http://gameinfo.na.leagueoflegends.com/en/game-info/get-started/summoners-code/
"Short is the joy that guilty pleasure brings." -Euripides Making games is our business, so it should come as no surprise that we want you to have a lot of fun. We want you to get excited, to have tension-filled moments, and to celebrate your success. This doesn't mean that we're okay with you ruining anybody else's day. Remember, taking a jab at your friend in the middle of the game is a lot different than making a glib remark at a complete stranger.
Someone who is unfamiliar with what you consider playful may take your comment as an attack and react unfavorably.
If two players on a team start fighting, good communication and teamwork become nearly impossible. Once communication breaks down, the likelihood of victory is drastically diminished. It isn't uncommon for simple, good natured teasing to spiral out of control into a loss, so do yourself a favor and don't run the risk of sabotaging your own success.
