Was the seedling more of a green color at some point - and if so, at what point did it turn yellow? Although it may be the photo, the color doesn't look great.
The main candidates are over or under watering. Also, it can be hard to keep pines watered appropriately in the soil mix you are using as it retains a lot of water. Ideally the soil around the roots will dry out a bit - not all the way - before watering again.
Could you also tell us if the tree is indoors or outdoors? Pines typically don't do well inside.