Why affects us the end of a relationship?

Your question isn't really clear but based upon the category you chose I assume you're asking:

What causes people to choose to end relationships?

1. They chose the "wrong person" for (yourself).

Oftentimes people become an "exclusive couple" or even get married while they're still in the "infatuation phase" of their relationship. They've never had any major disagreements/arguments or witnessed reaction to stress.

During the "infatuation phase" the word "no" is seldom if ever used. Everything you want to do your mate wants to do or so it seems. The reality is neither person wants to do or say anything that might blow their opportunity to be with the (new) object of their affection!

There's no need for (compromise) when both people agree!

However after there is an emotional investment or commitment many people then feel it's safe to reveal their "authentic selves".

This is when "differences" start popping up and we learn more about each others "deal breakers" & "boundaries" along with their fighting tactics.

You see them for who (they are) and you know if they're right for you! It's almost a cliché to hear someone say:

"He/She is not the same person I fell in love with."

Last but not least one's mate commits a "deal breaker" in their eyes which might be cheating, verbal/physical abuse, or whatever.

2. They got married/together for the wrong reasons.

Maybe there was an unplanned pregnancy, someone was tired of being single, all there friends were coupled up or married, one person joined the military is about to be deployed, had an "age goal" to be married by, or someone was given an "ultimatum" and they decided to appease their mate. However all along they knew deep down they weren't "all in".

3. They fell out of love or (stopped wanting the same things).

Relationships are like gardens in that if you don't nurture them they will silently die over time. Some couples put their all into raising their children and keeping up with their activities but leave little for themselves. Once there is an empty nest they realize the children were the only thing that kept them together. Essentially they have gone from lovers to "platonic friends".

It's easier to maintain a fire than it is to reignite a spark!

In other instances one person may decide they'd like to relocate or they have a job promotion opportunity that requires them to put in more hours or relocate and their spouse is against it. Maybe prior to marriage they discussed having children but after getting married one of them changed their mind.