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I think if you spent some time really dwelling on the basic dynamic of "pleasure" and suffering you would get the point. By just about all accounts, life has way more suffering than pleasure. Not many people actually contest this.
There is also a more extreme view that pleasure doesn't really exist at all, that it's just a neutral break from suffering. I am not sold on this, but I get the point. Think about the nature of alcohol, opiates, and endorphins, the description of their 'good feeling' is interchangeable with 'less pain'.
Combining that view with a belief that death is just eternal non-experience (which we all pretty much know, copes aside), and death is objectively much better than life. Now, that's not to say agreeing this requires you to be an anti-natalist, there are lots of other possible factors and value judgements to take into account.