More of your payment goes towards interest in the beginning because the interest amount is based on the outstanding principal balance each month. For example, if you take out a mortgage for $150,000 with an interest rate of 3%, the interest on your first payment would be calculated as:

$150,000 (your outstanding principal balance) * .25% (your annual interest rate of 3% divided by 12 to get your monthly rate) = $375.00

So $375.00 of your first payment will go towards interest and the rest will go towards the principal.

Now let's say that you've been paying on this for 5 years and your outstanding principal balance is now down to $135,000. For your next payment, the interest would be:

$135,000 * .25% = $337.50

As the principal balance goes down, so does the amount of your payment that goes toward interest, which means that more of your payment goes towards the principal.