Is forgetfulness & carelessness related?

When ppl "forget" to call me back, text me back, do a favor, or a promise, do I have the right claim that this person does not care? Overall I'm an utmost forgiving and understanding person, but the more it happens, the more I get frustrated. Is a simple reply "I'm busy" too much to ask? I know they care about me as a whole, but if u care about a person, wouldn't your goal be to not disappoint him/her? Or let that person down? Or am I being too harsh...