They feel like they are leaving one of their friends out or hurting their feelings.
First of all, realize that the duty of being in the wedding party is enormous. The Maid of Honour has a lot to do before and during the wedding itself, so you will want to choose someone that you think can handle it. If you’re the bride, you may feel that you have to choose a family member as well, but that’s not necessary. You might include them as a part of the party, but if you have a really close friend that you want to bestow the honour upon, then do so. Find someone who isn’t overly busy or committed. If they live in the town that you’ll be married in, that’s great too. Even if you’re out of town, you’ll get to be able to visit each other and she can make arrangements while you’re at your home. You also want to find someone who isn’t financially burdened. The thing is hen parties, manicures, tanning, etc all cost money, so you don’t want to stretch them or have to pay all their expenses other than their dress and hair and makeup on day.
And then the men
Just like choosing the Maid of Honour, you don’t want to choose someone that ‘should’ be the Best Man. This should be someone that you can count on to plan the stag party and get the other groomsmen to stay on schedule. Of course, your best friend may fit this bill, so the decision might be easy. Again, don’t pick someone who doesn’t have a lot of time or is financially strapped. Picking someone to be your right hand person is an important decision and an honour for the person that you ask. Although you may think that they are the right person, they may decline the offer and the extra work but don’t be offended.