Because his daughter should be a baby, yet she's the same age as Shulk, and Shulk's baby twins visit sometimes and they remind Chrom of the messed up timeline that confuses him. Before Shulk became a father Chrom worried that Lucina was getting too close to him...even though Shulk made it clear his heart belonged to his childhood friend back home and Master Hand forbade cross world crossbreeding/romances.
Why was the person who discovered cheese so keen on eating stinky solidified milk?