One minute it seemed your partner could do no wrong, and the next you find yourself gritting your teeth at the way he eats his cornflakes or tells the same jokes or never gets round to doing the things she promises to do.
The range of reasons a partner can be annoying is wide and diverse - and typically it will be the small things which annoy. And the next thing that happens is that irritability comes stealing in.
I have lost count of the number of times a client has said to me "I just find he drives me crazy" and "I don't like who I am becoming".
So what is going on here? Have you or your partner become someone else? Was love in fact blind? Finding everything your partner does irritating can be stressful, worrying and frustrating. And many clients talk of a sinking feeling that their relationship has become a burden, not a plus, in their lives.
Amid the disillusionment and doubt, is the fact that being annoyed can tip quickly into a more full-scale argument. Feeling annoyed, once articulated, quickly becomes irritability.
Irritability is contagious. Once the bad mood has been "caught" by both, then we have the metaphorical brimming bucket of water which is only going to take a few more drops before it spills over: "How come you never wipe the bench down" or "I can't believe you have lost the keys again".
The key question is, how frequently is this happening? If that brimming bucket is never properly emptied, then these irritations will continue to build and clashes between you will become part of your dynamic.
Steven Stosney in Psychology Today describes these cycles as "intimate relationship dynamics" - patterns in which both parties automatically relate to each other in set ways and are keenly sensitive to how the "other" behaves. Significantly, in these situations each partner is scarcely aware of their own behaviour.
Think about it: In the height of an argument it's remarkably difficult to objectively analyse your own behaviour. This is because only a tiny sliver of prefrontal cortex serves that purpose. In emotional arousal this sliver gets almost no blood.
Natural selection favoured recording the injuries we suffer over those we inflict. So what your partner says goes into long term memory - whilst what you say does not.
Once an argument has resolved, peace may be restored. Struggle and conflict in certain amounts are necessary and desirable in negotiating and renegotiating our relationships. If irritation and conflict are not entrenched in your relationship, then once able to think straight again, we can feel remorse and self doubt, kiss and make up.
But if the dynamic persists - the brimming bucket allusion - then that self-doubt, remorse and willingness to make up gives way to feeling chronically annoyed and irritation and resentment are never far from your communication patterns.
Trying to deal with this can feel too big, and avoidance is a common way of managing it.
But the reality is that avoiding the issue is not going to be of use if the issues are not addressed.
It is worth remembering that anger can build up over a series of disappointments. It is also worth remembering that with the honeymoon over, the very things you loved about the other may well be becoming the things that most irritate you.
The cute way he was so vague about what he had spent and her wonderful abilities to be so responsible about taking care of the bills, can become a predictable minefield: "He is irresponsible", "she is controlling", becomes the cry.
If bemused annoyance is becoming translated into an entrenched irritation, then creating opportunities to discuss is a must. Discussion does not need to be a confrontation but rather an opportunity for expressing what you want and finding out more about what your partner wants.
Top tips for addressing irritations
• Set a time to talk when neither of you is angry or emotional
• Be specific - many annoyances are behaviourally based
• Little things build up so it is important to work out what the issue is
• Use the "I" word... not "you are always late" but "I struggle when you are late"
• Collaboration and compromise is always a good idea
• Requests for change are not criticisms
• Allow yourself to be vulnerable and name what hurts
Persistent negativity and low mood, insomnia, and chronic lack of motivation may mean that one or other of you may be in a state of depression or anxiety. There may be contributing triggers such as ill health, lack of sleep or financial pressures.
It is really important to seek professional help if it is feeling too big to resolve on your own as a couple.
With the day-to-day irritations, however, remember that familiarity does not need to breed contempt - familiarity can breed compassionate understanding. And when that understanding is mutual, then the more secure will be our attachment, and with it a greater sense of wellbeing, which feeds back to the relationship. And, as with any significant human endeavour, it is a work in progress.