Some people hate you, others like, nay, desire you
EDIT: Sorry for the wait, I answered this originally on my phone between things during the holidays. Extended answer reasoning below:
I can ruin a party
Showing up uninvited
Having something (a twist ending to something you're invested in, a solution to a puzzle you're thinking hard about, etc.) spoiled for you when you don't want it can be decidedly un-fun
Though some are pleased to see me
When they get real excited
Even those who would first shun me
Won't mind meeting me again,
Or after certain indications
They'd be happy with me then
I'm lumping the rest of these together, because (to me) they all sort of point to a similar basic idea: spoilers are not necessarily always unwanted - they can be very welcome in certain circumstances. For example, if you were using a website where people posed puzzling questions, you might want to know an answer while allowing others to work it out in their heads without potentially accidentally reading the answer. Or, similarly, one person may be interested in a certain plot point of a movie or book just for curiosity's sake, while another may want to only come upon it in the course of reading or viewing on their own.
Should I tell you my nature?
Should I give myself away?
Alert and ready to oblige
When you tell me it's okay
This last bit makes me think of the nature of spoiler tags on online content - they allow someone to post the content, and for the content to remain in place, without it becoming visible without the active consent of the reader.