The aliens have less capacity for emotions than humans.Humans, or at least the Mews, are able to feel Mew Aqua. However, the aliens have to rely on other Mew Aqua samples, and find strong emotions incomprehensible sometimes.
Quiche has a mental disorderTieing to the first WMG. He's considered weird by the other aliens' standards, they call him weird on several occasions. On one occasion, he was able to feel Mew Aqua (episode 26).
The man Zakuro is seen crying over in a flashback was her fatherHer mother may have died before, and that's why she is closed off, doesn't have any parents seen, and became a model to fill in the gap of loneliness.
Pie has been on Earth before the start of series, to stop professor ShiroganeAfter all, during the final battle he showed himself in control of the Chimera Anima that killed the professor...