Why do interest rates go up when economy expands?

[Ivan Jouikov]

This has been on my mind for a while.

If economy is expanding, shouldn't there be more money flowing around, hence more people with money, hence more competition in the private lending arena?

Now I understand that the fed regulates "prime" % at which they lend to the big banks, so they lower the rate when the economy needs "easier" financing in order to be stimulated, but that has nothing to do with private lenders.

So, my question is - when the economy is expanding, shouldn't HML become Easy Money Loan?