The second is a bit deeper, where the song is directed to someone contemplating suicide. It describes the depressive state and then tries to convince the person that "it's a livin' thing; it's a terrible thing to lose".
The third one is in reference to a still-born. The first verse is the conception, the second the birth, and the third, the depression coming from losing a child before it was even born.
Alternate: abortion. The second verse is the abortion and the third is regret.
It's amazing how open this song was left to different interpretations. The lyrics support at least three different interpretations.
The first is the cliché classic rock song interpretation: drugs. It's very common and easy to attribute. The song describes a trip, how the drugs start to influence your life, and holding on to those drugs thinking that you need them (particularly from the line "when you look back in time to your first day")
The second is a bit deeper, where the song is directed to someone contemplating suicide. It describes the depressive state and then tries to convince the person that "it's a livin' thing; it's a terrible thing to lose".
The third one is in reference to a still-born. The first verse is the conception, the second the birth, and the third, the depression coming from losing a child before it was even born. Alternate: abortion. The second verse is the abortion and the third is regret.