I have noticed this in myself and in many autobiographies as well. Some memories tend to stick out really well even though there appears to be nothing special about them. For example, I vividly remember walking into my babysitter's house once when I was a child. There was nothing special about that day from any other and yet I can remember it much better than some more intense memories I've had of getting hurt or being really happy. In books as well, authors often recount plain memories that one wouldn't expect to stick out. Is there some other factor(than our emotional state at the time) that is involved in making some memories more memorable?
First, whenever talking about memory, it helps to be specific. Memory is, somewhat controversially, categorized in a number of different ways.
I'll take the liberty of assuming that you are specifically referring to episodic memory. Given your examples, we might even call it autobiographical episodic memory.
Episodic memory is special in that it is a highly multimodal kind of memory. It's the kind of memory involved in "mental time travel", or the ability to retrieve a memory over many sensory domains and "relive" it.
As such, there's a lot we don't know about why some of these memories are encoded more strongly than others. This is especially the case for events that we don't remember paying particular attention to, like walking to your babysitter's house.
It could be, as you mentioned, that the emotional state associated with that event is strong. Or, it could be that there's some combination of sensory inputs (sight, smell, taste, etc) that affect our bottom-up attention and are involved with being able to remember the event (Cabeza, 2008). It could also be that particular dynamic brain states are simply more "friendly" to encoding these memories (Watrous 2014), and your brain may have been in such a state at the time.
It's a very open question. If interested in this kind of memory, I would recommend reading Elements of Episodic Memory (Tulving 1983).
Cabeza, R. Role of posterior parietal regions in episodic memory retrieval: the dual attentional processes hypothesis. Neuropsychologia 46, 1813–1827 (2008).
Watrous, A.J., Ekstrom, A.D. The spectro-contextual encoding and retrieval theory of episodic memory. Front Hum Neurosci, 8 (2014)