You have had a long and tiring day. It been a difficult week. You get grumpy and irritated and you need someone to take it out on. Who is there to oblige? Your partner of course. Always available and it's surprising you're feeling bad, so what do expect?
What they expect is that you would treat them nicely. If was your colleague that was standing when you walked in, you would manage to be polite them, so why not your partner? After they should be the most important person in the world to you, so why don't they the best treatment?
It's so easy to use your partner as a handy Sponge to absorb all your anger. But it is not right. I have seen pleeof couples who are grumpy and snappy with each other regularly or down right rude, simply because they can't be nice. Not because either has done something wrong.
What's wrong with simply saying nice things to each other? What became of 'please', and 'thank you' and 'would you mind'.? If you want to feel really positive about what you have together, you need to start by being courteous and respectful to each other. Remember your basic manners and speak with respect and kindness to one another. Fix them their favourite, give them gifts for no reason, because you love them. Compliment them, help them with the unusual job, even if it's not yours to do it.
If your partner comes home after a tiring day, don't give them a chance to take out their irritation on you. Make them a drink, ask how they are and how their day went and listen to what they say. Be interested. Tell them to shower and rest, while you fix them something to eat, or do the remaining task they are supposed to do the day. Do this because you care.
If you have children, what better example could you set them? Think about it. You're asking to be treated in the way you treat them, so you had better make it good. But that's not why you're doing it. You're not being nice in order to make them be nice. You're being nice because you love them and that's what they deserve.