Why People Choose Living Together and What Happens When They Do

There are many reasons why people choose to live together, the most popular being the one I just mentioned.
Here are some that represent real life situations I have personally seen.

Example 1
One partner feels love for the other, but the object of their affection will not commit to getting married. They first person assumes that if they can live together, eventually the person who is unable to commit will eventually do so.
Sometimes this happens, but more often than not, it doesn’t.

When the couple does eventually marry it rarely is because the non-committal partner has a change of heart.
It more likely is because
It is easier to tie the knot than to keep arguing about marriage,
the partners have aged or become ill and thus have fewer options,
has become more convenient or financially beneficial or
children have become involved and it is to their benefit for the couple to marry.
Whatever the reasons, the spouse who did not want to commit always carries some level of resentment because they feel that they have been coerced into marrying. In many cases, these feeling fester, damage the relationship and cause it to end.

Sometimes couples live together because one or the other of the partners is unable or unwilling to commit to getting married. | Source
Example 2

A young couple is in love but each is struggling financially. They think that if they move in together, it will cost them less to live and allow them to save for the big wedding they have dreamed of having.

This may seem a good idea, and sometimes it works. However, once people start living together they see sides of each other that are not always the most pleasant.

If they are mature and are able to accept the flaws in each other, they usually will be able to follow through with their plans.

If not, the relationship will fall apart.
They may find, for example, that one is a spendthrift while the other is not, one is neat about his habits but the other is a slob, or one of them has an addiction that negatively affects the relationship.
While it is better to learn about these things prior to marrying, it also creates the risk of losing the entire relationship due to the ease of dissolution. Had the couple married, they would have been more likely to attempt to work on resolutions to their problems.
Example 3

A divorced older man becomes involved with a much younger woman. She is attractive and pleasant to be with, but he doesn’t love her. She, on the other hand, thinks it would be nice to move in with this man because it would save her a lot of money. She might even be able to talk him into marrying her. However, he has vowed never to marry again and does not want to have another person involved in dealing with his college aged children.

He asks her to move into his home and allows her to do some decorating so that she can feel comfortable. It is much easier to have a built in companion at his age than to start dating, so the expense of keeping this woman in his home is worthwhile to him.

As time passes, the woman becomes increasingly insistent about marriage, but he absolutely refuses. Now they have created a contentious living environment, but he can’t just “put her out” due to feelings of guilt, and she left with nowhere to go!

If this situation continues long enough, the woman has given this man the last best years of her life and has greatly limited any chances she may have for finding a new relationship that might end in marriage.

He on the other hand, must tolerate the conditions he has created, but allows them to continue because he feels he has no other choice.

In the end, living together has given each person some benefits but has also created a great deal of permanent discomfort.

I know one couple who continued on this way for more than 20 years. She finally had enough and left him. A year later, he married another woman!.

source http://tasboy.com/why-people-choose-living-together-and-what-happens-when-they-do/