Why do we Define relationship term as long or shor

I have thought about this a lot lately on why we are so hung up on what type of relationship we are looking for. Some of us are looking for short term, many are looking long term. It seems we use this many times to determine criteria for getting involved with someone.
I remember when my wife and I were first dating, she was never going to get married again, and I was looking at marriage, settling down and having a family. In a way this initial concept of our relationship defined much of our marriage. I was always planning the future and never paying to much attention to the day to day details of life. My wife on the other hand always lived in the here and now, never really giving the future to much of a thought. It was both of our shortsightedness that damaged our marriage beyond repair. Why I planned away our future she became bored, dis-satisfied and started looking elsewhere to fulfill those day to day needs. My head was so far into the future I didn't even realize our marriage was fractured until it was far to late. My wife caught up in the grind of day to day life could never see into the possibilities of our life together.
Now, left to decide what I want from life and reflecting back on the past I am faced with this question. What do you want long term, short term. But, instead I have had an epiphany of sorts. Why not just let the dynamics of any relationship steer the course for you. What do I mean by that? The following poem really defines what I am driving at.

By: Brian A. "Drew" Chalker

People always come into your life for a reason, a season and a lifetime. When you figure out which it is, you know exactly what to do.
When someone is in your life for a REASON, it is usually to meet a need you have expressed outwardly or inwardly. They have come to assist you through a difficulty, or to provide you with guidance and support, to aid you physically, emotionally, or even spiritually. They may seem like a godsend to you, and they are. They are there for a reason,you need them to be. Then, without any wrong doing on your part or at an inconvenient time, this person will say or do something to bring the relationship to an end. Sometimes they die, Sometimes they just walk away. Sometimes they act up or out and force you to take a stand. What we must realize is that our need has been met, our desire fulfilleed; their work is done. The prayer you sent up has been answered and it is now time to move on.
When people come into your life for a SEASON, it is because your turn has come to share, grow, or learn. They may bring you an experience of peace or make you laugh. They may teach you something you have never done. They usually give you an unbelievable amount of joy. Believe it! It is real! But, only for a season. And like Spring turns to Summer and Summer to Fall, the season eventually ends.
LIFETIME, relationships teach you a lifetime of lessons; those things you must build upon in order to have a solid emotional foundation. Your job is to accept the lesson, love the person/people (anyway);, and put what you have learned to use in all other relationships and areas in your life. It is said that love is blind but friendship is clairvoyant. Thank you for being part of my life.....

I guess my whole point in this is to go into any relationship without any preconceived notions of time. Rather look at the relationship in the terms of how can I grow from this, what can we share, what can we learn, and most of all what can we experience that without the relationship we could have experienced.

Thank you and may whatever God you pray to, bless you with good people to have a relationship with.