As soon as you announce your pregnancy, people are quick to tell you how hard it is to be a parent, how little sleep you'll enjoy, and how overwhelming this new responsibility will be. However, babies are pretty easy when compared to the emotional warfare that is caring for a toddler. Small children are exhausting and the repetition of daily chores can make the happiest of partnerships falter. Little irritations build up until you end up having one of the fights every couple has when their kids are toddlers.
When your toddler is pushing you to the limit of your patience and making you want to open the window and scream into the wind, you're bound to start taking things out on your partner. Many toddlers also still exhibit some disordered sleeping patterns, so parents are often sleep deprived, too. Throw in career demands, household chores, and different or conflicting parenting styles, and it becomes easy to see why the toddler years are especially challenging for relationships.
I found the year between my son turning 1 and 2 to be the most difficult. He wasn't a baby anymore. Instead, he was mobile, could easily get into mischief, and was starting to become more defiant and independent. However, he didn't have the language skills to explain exactly what he wanted or why, which culminated into a shared frustration that laid the foundation for the following arguments. Sorry, honey.