Xtopher

Tyrion knows for a fact that Balon Greyjoy hates the Starks because they defeated the Greyjoys (actually humiliated and massacred) during Robert´s rebellion. Thats why Balon had to bend the knee and accept Robert as king and give his son Theon to the Starks. Balon has always hated the Starks for it so Tyrion´s idea to marry Myrcella to Theon was actually pretty clever.