And it's not like you can just coast when you are in a stable relationship, either. You have to keep them interested in you, spend a lot of time with them, work out the inevitable disagreements and conflicts that happen between you two, between you, your partner, family, friends etc. Then over time both you and your partner grow and change, and you have to adapt to that too. Before I got into my first relationship, I thought "oh what a relief I finally have a boyfriend, I can just coast it out" and I quickly saw I was dead wrong.
It's time consuming to maintain relationships, especially long distance ones or over very long periods of time or both. How anyone can do it with multiple people without emotionally carking out is anyone's guess.