Eating the last of something without asking if the other person wants some

Hi galls. I grew up with 2 siblings and was always thougt that if I wanted the last of something I should ask my siblings if they still want any and if the case is yes, split it. Like a slice of cake or something.

My bf grew up a single child. He just goes around and finishes everything. I keep telling him I dont apreciate it bc as far as I know its just rude but he still does it. He keeps sayong what does it matter ill just by a new one. But that doesnt work out bc of reasons like this.

He fineshed my fav treat last night I found out when i wanted some this morning before the stores opened. So I cant get it. When he wakes up he says oh btw i finished your treat but ill go get new ones if youd like. But yesterday we litterly had a discussion about how we need to save more money and spend less on things like this so I was plannning on saving some a little longer so I can keep enjoying it. And on top of that we are both very sick right now and since its currently minus 2 degrees neither of us should take a 30 mins walk to the store. So no. We cant get new treats.

Am I crazy for saying its not nice of him to just finish shit without asking? Like we have our own personal treats ( I have my chocolate he has his chocolate milk, occasionally I take a cup of his milk but if he finishes that im cool with it bc its his thing idc for it) but when it comes to the things we both enjoy a lot should or should he not ask before finishing things?

Wether im wrong or right I still think its reasonable for me to be upset bc he never said he doesnt want to ask or that he doesnt agree. It always just goes like this

He finishes something I tell him that upsets me he says oh im sorry ill buy a new one i try to explain thats not the point and then he just agrees yet DOES IT AGAIN.

But maybe i should still change my views on it. We've been together for more than 2 years now, living together and having our finances together for almost 2 years as well and this has been bothering me for 2 years. I have not once in those 2 years finished anything without first asking unless it was something that was just mine.