I might fib about it if I knew he was going to make a sarcy comment about how much I’d spent.
This, I see it on mumsnet and in real life all the time. People judge, we all do, it happened inadvertently in our minds before we can stop ourselves and not everyone can stop that showing even if they want to. Maybe you're like that OP, and you don't even realise, and he doesn't want to deal with the eyeroll/sarcy comment/having you bring it up as a funny story about him.
Especially in cases where it's not shared money and the other person isn't affected. Why lie in that case?!
See all reasons stated above. I've lied to a partner before about how much i spend on high end make up. It was my money and he'd make what he genuinely thought were funny comments (no malice at all) about it sometimes. Sometimes i just didn't want that so would tell a white lie. And he'd do exactly the same to me about gaming.
Liars lie, right? It's literally who they are
This is so overdramatic. Everyone lies. Is everyone a 'liar'.
'your jeans looks nice'
'no of course your haircuts not bad'
'yes this meal you've made me taste's delicious'
'I loved the present you got me'
'yes mum i always iron my clothes' (lies, i only iron when im going to see her)
'yes of course i researched 20 different toasters before i got this one dad' (lies, i bought the first one that had decent reviews because im not retired like him and can't spend hours researching the best toaster but i can't be arsed for the lecture from him about 'always doing your research')
we all lie to children all the time (santa etc)
Lying can be polite, it can sometimes be just because the truth isn't worth the grief, it can actively bring some people comfort.
Everyone lies, everyone is a liar, doesn't mean everyone lies about the big things.