Surprises are Culturally Normalized Violence
We are all intentional beings, we must exercise our agency to feel empowered in our own lives and to actualize our truest sense of self.
As intentional beings, when we meet each other out in the world, we must respect that we are co-creating this life by respecting each others choice and freedom to do as we will. To, by word or action, disallow or invalidate the choice of another being is to do violence to them.
When we plan a surprise for someone, we remove their ability to choose. We can get lost in our own feelings about the surprise, the look on their face when, how meaningful we think the gift will be to them (our to our self), thinking they will love this.
You may have no idea what traumas someone has or what triggers lie undiscovered, they may not even know themselves. You have no idea what landmines you may step on that will hurt them, which is probably not what you intend.
As a culture in the West, we normalize being grateful for gifts, being grateful for someone expending resources on your behalf, especially if you didn't ask for it. Your choice is removed by surprises because you were never even given the chance to say no, or even yes!
Spoil the surprise. 
Always ask if someone would like you to get them that thing or plan a special event for their occasion. Don't assume, a surprise is never about you, it's about the person you may hurt. 
