Note: This question was supposed to cover all "why someone does X" questions, not only the example mentioned here.
After fixing the formatting a bit, and reading twice, I feel this question should be on-topic. This question is asking about "why my girlfriend is mad at me after I do ..."
While that type of question is inherently opinion-based (we cannot possibly know what someone's thinking and why someone does something), it seems the reason is rather obvious to reasonable people, but not to OP.
This answer beautifully address the reason why his girlfriend surely will be mad at him, and I believe this is the fundamental reason why she's mad. This is a very helpful insight to future visitors who might do the same thing like OP (which I believe they will be a lot of teenagers).
I think questions that ask insight about why someone does something should be on-topic. The nature of this type of question is opinion-based, so it must be handled on a case-by-case basis. So, while it is on-topic, it may be unanswerable because too opinion-based.
Q: How the community feels about "Why someone does X" questions? How should we handle them?
Note: For this particular question, I think it is on-topic, and while opinion-based, I think this is answerable, and that answer gives an example how it should be answered.
Another example of good "Why someone does X":