I don't glorify pain, it is not an end in itself, but I do embrace pain and discomfort, because I know it is not just one, but indeed the only path towards pleasure, which is my goal and guides my selection of which pains to embrace and which to shun (because it either does or doesn't outweighing the pain previously endured to attain it).
I will try to answer Cassius's initial question by showing that pain is not the only path towards pleasure. Julia Please don't take what follows as a criticism of your position. I think hedonic calculus is not only subjective but strictly personal and it's everyone's responsibility to make the calculus as efficient for themselves as possible. Whatever works for you, I'm happy for you and it's not my business or intention to criticize. I will just present my position which happen to be different to yours.
Some years ago, I started to realise that it's rather easy to appreciate pleasure when it comes from pain. In other words, when we move from discomfort to comfort, it's rather obvious to most people that this is a good time to feel good. I also started to play with the idea of pleasure as the ever present background to life. The background which is largely neglected by people. It's easy to detect pleasure in a glass of water in the middle of Sahara but awfully difficult to detect pleasure in a glass of water standing at the bottom of crystal clear waterfall with infinite supply of refreshing water at our disposal. The abundance makes people indifferent.
To my understanding, by observing that the absence of pain is pleasure Epicurus removed 'neutral state' for the purpose of tuning people into pleasure that is provided by nature in abundance. In other words, to teach people to feel pleasure when they feel nothing because feeling nothing is missed opportunity due to our misjudgement of reality we find ourselves in.
So my answer to the question: Why Do We Consider The Absence of Pain To Be Pleasure? is this: To gain ability of noticing and thriving in abundance of pleasure available to us instead of neglecting, being overwhelmed, being ashamed and ultimately rejecting pleasure that doesn't come from pain. After all, we are trained to firmly believing that there's no such thing as free lunch, right? Free pleasure must be repulsive. Earn it by suffering or reject it completely. Well, no. Pain is not the only path towards pleasure. It's definitely one of the paths. We cannot avoid pain. It will come whether we like it or not. We should learn from pain how to deal with it and how not to be broken by it, sometimes even how to transform it to pleasure but mostly, we should realise, appreciate and learn how to live our lives unashamedly in pleasure.