It'd be great if you could freeze time and take all those wonderful friends you have and keep them just as they are. Forever.

But as we know, friendships change. Rarely is a friendship ever set in place, perfectly complete and without some conflict. There are probably friends you get along with better than others, and even a few that you would consider as "best" on the friendship ladder.

If these friendships are so great, why don't they last?

Research shows that most of us replace our friends, intentionally or not, fairly often. Live Science says that "when it comes to your close friends, you lose about half and replace them with new ones after about seven years."

Here are a few reasons why this happens.

No Formal Ceremony to Bond Friends Together

In other relationships, like marriage, there is a ceremony that bonds people together legally and emotionally. Friends and family of the couple get involved and may help support them. It's not as easy to completely shut out a spouse and just leave, because you have to formally declare that you're done. Even then, you need to legally divorce yourself from the other person.

In a friendship, however, there's no formal commitment. People become friends in an instant, rather than through a legal ceremony, and their relationship can be ended without giving a reason why. This is both what makes friendship so remarkable and so frustrating.

Other Relationships Take Priority Over Friendship

In an ideal world, you can balance all your relationships, from your friends to your kids to your parents.

In the real world, there are many times when your family needs you and as a result your friendships will suffer. This happens when you're just busy in general, and also when there is a traumatic event, like an injury or family crisis.

Friendship needs to be flexible in order to survive. The flip-side is that without proper nurturing a friendship will definitely end. It takes a special set of friends that can say to each other "I don't have time for you now" and then go back to where they left off when the crisis or family time eases up.

The problem is that by its very nature, people get an emotional benefit from friendship that they enjoy and in some cases depend on. This means that when a friend has to put you second, it hurts. It also leaves you without an important part of your support system.

What's more, you'll think about all the times you put that same friend first, and you'll feel resentment.

Change In Lifestyle

One of the most common reasons great friendships don't last is because our lives are constantly changing. We may get married, have kids, move, get a new job, or some other big event and as a result it changes both us and our friends.

Even if your friend experiences the same thing as you (marriage, for instance) at the same time, there might be a difference in the way you each look at life from that point on, or a change in the things you're experiencing. Some friendships can handle this change. Those are the ones where both people involved have worked through some conflict and managed the changes well.

In most other friendships, however, change means new ground that can draw a wedge between friends, even when their relationship is seemingly good on the surface.

Friendships might be temporary at times, but each friend you meet can add a lasting lesson to your life. Enjoy your friendships for what they are and for however long they remain in your life.