Many researches have been done on what make a couple happy on the long term and one of the most interesting findings is about what seems to be a sure recipe for the end of a relationship.
These short list of killers of a relationship was develop by Dr Gottman and goes under the name of the four horsemen of the apocalypse:
Criticism is the first one. When what the other does or say is never good enough. The antidote is to express yourself with 'I' messages and therefore talk about what you think and feel.
The second is contempt, that sense of superiority that drives you to behave as if the other is a lost cause. The antidote is reinforcing the positive, be grateful and express all that is good.
The third is defensiveness and it is about viewing oneself as the victim and the other as the responsible one. The antidote here is to take responsibility for how you contribute to the situation.
The last is stonewalling and it has to do with stepping out of contact, withdrawing from communication with the other. The antidote is about taking for some time to regroup and come back to engage when ready.
If you want to see if any of these apply in your relationship, make sure you question yourself not your partner, make sure you don't do any of these.