Negativity within a relationship is almost inevitable but, of course, there can be differing amounts of negativity. A little negativity in a relationship is probably expected and can be dealt with or even ignored. However, when there is a lot of negativity within a relationship, this is harder to deal with.
The first thing which negativity in a relationship can do is to cause more negativity, especially if any issues are not resolved.
Within my own relationship, there is, I feel, considerable negativity. This began fairly early on and, rather than being able to do anything about it, it has continued, got worse and now the relationship feels almost stale.
I blame my partner in the main as he is the one who creates problems. Of course, I will complain about them and he, in turn, complains about me complaining. It goes around in circles and nothing gets resolved because, while I can and do stop complaining, he is still continuing to do all of those things about which I complain.
As we have children, they are affected by this negativity. We get angry with each other and within situations where we should not really become angry. Our children see that we spend all of our time doing different things and never doing anything together. It gets harder and harder to spend time together because we spend so little time together that any attempt to resolve any problems seems that we are only spending time together to sort out problems, leaving no time for "quality" time and giving my partner another excuse not to spend time within the family.
The effects are, of course, not intentional but they certainly exist. Negativity breeds negativity, more problems are caused than are solved because no one wants to be negative so we almost never talk about problems and we never sit down together and just talk because we know there will be arguments. Of the little time we do spend together, I am so often afraid to talk because there will be arguments so we will just sit watching television and, once I have gone to bed, my partner will stay up to play on the computer.
Then there are lies which are told as, I think, a result of the original negativity. Basically, one partner is unwilling to try to resolve issues and, therefore, decides to find some way of avoiding them which would often lead to lying as they know they are doing the wrong thing.
All of this negativity within the relationship puts a strain on the relationship itself, particularly for the one who is trying to deal with it while the other is doing their best to avoid it. It has also caused a lot of stress and then, in a relationship where children are involved, it affects them because they don't see that Mum and Dad love each other, they only see them fighting.
In my case, I am the one looking after the children while my partner goes to work. He will come home and do as he pleases which causes more negativity as neither myself nor my children understand why he does this and we will be upset by the situation. In our case, the negativity has also caused a financial strain because my partner "deals" with most situations by going out drinking showing that he does not care which, again, causes yet more negativity.
Basically, negativity in a relationship can cause all sorts of problems and it is best to resolve them as soon as possible. If only one person is prepared to make any effort to resolve the issues then the other person does not care and does not deserve to be in that relationship.