Complex sentence: He is supportive of his friends'engagement, but ultimately realizes his lingering feelings for Robin would make it difficult to be around them once they're married, and so plans to move to Chicago the day after they get married.
Split into simpler sentences: He is supportive of his friends'engagement, but ultimately realizes his lingering feelings for Robin would make it difficult to be around them once they're married. He plans to move to Chicago the day after they get married.