Why do you love it? I'm honestly curious as to what it does that SMS, Messenger or other messaging apps don't do.
I can have group chats in WhatsApp and send video, images and sound. Plus I like how it lets me know if messages have been delivered. It's also fast and doesn't drain my battery mercilessly. The times I've tried Skype on my mobile device I can see my battery meter go down as I type.
But mostly I use it because all of the people I generally interact with in real life are on it. All of my family, friends and acquaintances use it.
I like how you nailed the reason for Facebook buying WhatsApp.
It just works perfectly without ads, slowness, UI changes.
It's simple to use, my mom & dad uses it easily (>60).