The last lines of the article:

So is death bad for you? I certainly think so, and I think the deprivation account is on the right track for telling us why. But I have to admit: Puzzles still remain.

Kagan does feel that death is "bad", but he only throws this in at the very end after spending the entirety of the article arguing the opposite.

One of his dominant questions is: Why do we feel bad about the loss of time after our death as opposed to feeling bad about the loss of time before our birth. I won't go into detail here about the article's content, but I do have a thought about it.

This is just me running with an idea in the moment, so I apologize if it is not well organized:

Let's say we have just purchased tickets to a concert. It features a band we have always wanted to see play live and the concert is several months away. We may certainly feel impatient and agonize over the wait, but in some sense the anticipation is a build-up to the inevitable moment of pleasure we feel when the actual day arrives, followed by the actual moment when we are at the concert hearing the band play in a crowd of people. Once the concert is over, it is over in every sense. The anticipation--having something to look forward to--is over, AND the event itself is over.

If we look at being born and subsequently dying as though they are similar to buying tickets to a concert and attending the concert, I think we can define why the time before the concert is not perceived as "bad" but the time after the concert has ended could certainly be percieved as "bad". Before we are born, the events of the world can be perceived as the build-up, the anticipation phase, or "buying the ticket". The world is being prepped for our entry. Life itself is the concert, it is the show we all want to be a part of.... we want to be in that crowd hearing the music. When the concert is over, there is an inevitable sense of loss. Everything leading up to the concert fueled the ultimate enjoyment of the concert itself. What comes after the concert can only be seen as "less appealing", or "bad" in comparison to the build-up to and excitement of the concert itself.

In other words, we see the events leading up to something we "want" as being positive, even if they present some level of agitation due to impatience or a strong desire to just get there already. We inherently know that the waiting will make it all that much sweeter. Yet the end of something we "want" to continue is difficult to define as anything but "bad".

Being upset about the time lost BEFORE our birth would be like being upset about missing a concert we never wanted to buy tickets for in the first place.