While I agree almost entirely with Gareth Rees' answer, I think it can be made simpler with a bit of cultural context. Aragorn is a king. He's in a position to command, and while he's not the king of all Middle Earth, he has claimed the throne of Arnor as his birthright, which includes the part of Eriador that the Hobbits call the Shire. Pippin is a high noble of the Hobbits, the heir of the Thain of the Shire.

If Aragoron had said to Pippin "go now, with my thanks. You've served me well, now return to your Shire and live as you will", then, leaving aside the material benefts that Gareth Rees discusses, it would have been quite an insult to Pippin and to the Hobbits! A dismissal would have been a statement that Pippin had nothing to offer that Aragorn could want, that Hobbits were irrelevant, and that the Shire should go back to obscurity and not get involved in the affairs of the big people. Instead he's saying "we've defeated Sauron, but there's been so much destruction, and now it's time to rebuild the world. I want you, my trusted friend and sworn servant, to help me with that job, and in return, I will make sure that you, and the Shire, benefit from it." That this comes as a solemn command, with no option of refusal, well, that's just part of the cultural trappings of feudalism — but it was still a mark of trust.

While I agree almost entirely with Gareth Rees' answer, I think it can be made simpler with a bit of cultural context. Aragorn is a king. He's in a position to command, and while he's not the king of all Middle Earth, he has claimed the throne of Arnor as his birthright, which includes the part of Eriador that the Hobbits call the Shire. Pippin is a high noble of the Hobbits, the heir of the Thain of the Shire.

If Aragorn had said to Pippin "go now, with my thanks. You've served me well, now return to your Shire and live as you will", then, leaving aside the material benefts that Gareth Rees discusses, it would have been quite an insult to Pippin and to the Hobbits! A dismissal would have been a statement that Pippin had nothing to offer that Aragorn could want, that Hobbits were irrelevant, and that the Shire should go back to obscurity and not get involved in the affairs of the big people. Instead he's saying "we've defeated Sauron, but there's been so much destruction, and now it's time to rebuild the world. I want you, my trusted friend and sworn servant, to help me with that job, and in return, I will make sure that you, and the Shire, benefit from it." That this comes as a solemn command, with no option of refusal, well, that's just part of the cultural trappings of feudalism — but it was still a mark of trust.