Ghouly, just repeating in the right spot, you need something that throws characters together. Because currently, its like we are all writing separate stories.
Idea's:
1. The lesser of two evils
2. Politics (e.g Tau high command want you to help the eldar because of a trade agreement.)
3.Grudging friendship/ rivalry.
And I'd also like more info on the planet, e.g is the north pole an icy plain, or frozen jungle?