Relationships

Why is breaking up so hard to do? And why is it the nobody realizes how hard it is until they're doing it?

Two people in my life are ending some pretty long term, serious relationships. One is a co-worker friend of mine (age 25), who had been w/his girlfriend for about 7 years. They've been having some problems lately. Mainly they're realizing that they've grown into two completely different people from the ones at the start of the relationship...two people who aren't compatible anymore. So he's going through that right now. And to boot they just bought a house together in May of this year. So she's moved out and he's working on renting it out for a few years in order to sell it.
He thought he was going to bounce right back from all of this. Split, take care of business, and then be fine. I tried to tell him that clean breaks are good, and make the getting over things easier...but things still aren't that easy to get over. He didn't believe me...I think now he does. I feel really bad for him, but being married it's a little weird trying to reach out and be there for him. I don't want it to seem to anyone that i'm interested in him...b/c that's not the case. I just know what it's like to get out of a serious relationship...so I can understand a little bit of how he's feeling. I just figure he might need someone to listen and talk about things w/. I think I'm doing pretty well at not coming on too strong about it. I think I'm putting the right amount of effort into it to show that i'm just here for moral support. Hopefully i'm right in my assumption.
The other person is my sister-in-law. She and my brother-in-law (husband's brother) are going to be splitting up soon. He has cheated on her several times, and once even had a full on relationship with another woman. How she has put up with antics for 10+ years is beyond me.
Anyway, she finally told him that he either cleans up his act or she's leaving him. Well he decided that he wasn't going to clean up his act...she said that as soon as she found a new place to live (there lease is just about up) it would be over. It seems to me that he is getting his cake and eating it to. I get the impression he considers them over, but if she's going to give him the benefits of a marriage w/none of the long term commitment...he's all for that. I am trying to get her out in the dating pool. Not so much as to find a new boyfriend...but to help her feel desirable, worthy, and wanted again. I'm not expecting her to find anyone new anytime soon...I don't know how smart it'd be...but I am hoping she has some fun and does some stuff that makes her feel good. But she won't do anything, or really talk to anyone, or even meet them while other people are around until she's not living with her husband. I tell her that I feel it's not wrong for her to meet people in a group setting. Then it's just like friends hanging out. She says that while her husband may not be being completely faithful right now, he still does stuff that shows he cares...so she wouldn't want to do that to him. I get it, but on the other hand I don't. That's why I said earlier that clean breaks are best (is that what I actually said? if not i should have)!! This break is messy and very drawn out.
I guess not all things in life can be simple. I just worry that she's disillusioned and thinks that he's going to not want it to end at the last second...and so she's holding out. I've talked to him and he's ready for the split. I just hope for the best for her.
Anyway, that's my stories about two different types of break ups and how it's a hard thing to do. Best of luck to both of the parties involved. And best of luck to everyone who's trying to have a relationship now-a-days. It's hard what w/the economy being so poor...causing stress to a lot of people. And so many other factors. Best of luck to us all...