Why are there always some unconfirmed transactions?
Transactions selected in mining blocks is not like a queue, where you wait your time and get to the front eventually.

Fees are not mandatory, but should be included in every transaction, if you have sent none, or low fees, you can still be selected (although it's highly unlikely), but the miner would be silly as they could just take the higher fee offered by someone else. Remember when you post a transaction you offer a fee.

If you owned a bakery and made 1000 loaves of bread a day, but had 1500 customers, would you sell them to someone offering $0.10 when other people are offering $1.00+.