Why does the amount of interest change each month?

Every month you make a monthly payment, and then every month the interest is added on for that month. Some months have more days than others and so some months it will be a slightly higher amount of interest because there have been more days in that month for the interest to accrue.

The interest on your loan does decrease, but if you're near the beginning of your loan you won't see a dramatic difference until you've paid a substantial amount off your balance.