I agree with the content of the answers of <PERSON> and <PERSON>, however, aside from the content of what he said, I think it's also important to address <PERSON>'s tone here. In particular: He's joking.
<PERSON>'s 'harsh' tone to <PERSON> here is firmly tongue-in-cheek. His statement is a humorous way of expressing his appreciation for <PERSON>'s service to him, to Gondor, and to the mission of the fellowship, as well as and his desire to continue their friendship and for them to work together again in the future. He is indeed expressing the value and importance of <PERSON>'s service to Gondor as the other answers point out, but he's just using a humorous tone to do it. His firm and harsh tone is verbal irony meant to humorously contrast with the gratitude and respect he's actually conveying.
If there were any doubt about how <PERSON> felt about the four hobbits, it should be completely dispelled (in the movies, at least) in another line when he's being completely serious:
My friends, You bow to no one.
While that particular scene isn't in the book, <PERSON>'s respect for and gratitude to the <PERSON> certainly is. At the beginning of the chapter Many Partings, <PERSON> goes to <PERSON>, who tells him:
'I know what you have come to say, <PERSON>: you wish to return to your own home. Well, dearest friend, the tree grows best in the land of its sires; but for you in all the lands of the West there will ever be a welcome. And though your people have had little fame in the legends of the great, they will now have more renown than many wide realms that are no more.'
A few paragraphs later, <PERSON> further tells <PERSON>,
'But now before you go I will confirm the words that <PERSON> spoke to you, and you are made free for ever of the realm of Gondor; and all your companions likewise. And if there were any gifts that I could give to you to match with your deeds you should have them; but whatever you desire you shall take with you, and you shall ride in honour and arrayed as princes of the land.'
<PERSON>'s statement to <PERSON> was but one of many examples of tongue-in-cheek dialog <PERSON> employed in the Lord of the Rings books. 381
As <PERSON> mentioned in a comment on another answer, another example of this, also from Aragorn, was his 'punishment' of <PERSON> for spilling blood in the Hallows and leaving his post without leave (in order to save <PERSON>'s life from Denethor):
And the King said to <PERSON>, '<PERSON>, by your sword blood was spilled in the Hallows, where that is forbidden. Also you left your post without leave of Lord or of Captain. For these things, of old, death was the penalty. Now therefore I must pronounce your doom.
'All penalty is remitted for your valour in battle, and still more because all that you did was for the love of the Lord <PERSON>. Nonetheless you must leave the Guard of the Citadel, and you must go forth from the city of Minas Tirith.'
The the blood left <PERSON>'s face, and he was stricken to the heart and bowed his head. But the King said:
'So it must be, for you are appointed to the White Company, the Guard of Faramir, Prince of Ithilien, and you shall be its captain and dwell in Emyn Arnen in honour and peace, and in the service of him for whom you risked all, to save him from death.'
<PERSON> provided us with more amusing tongue-in-cheek dialogue a few pages later between <PERSON> and <PERSON>:
And before he [<PERSON>] went to his rest he sent for <PERSON>, and he said to him: '<PERSON> son, have you your axe ready?'
'Nay, lord,' said <PERSON>, 'but I can speedily fetch it, if there be need.'
'You shall judge,' said <PERSON>. 'For there are certain rash words concerning the Lady in the Golden Wood that lie still between us. And now I have seen her with my eyes.'
'Well, lord,' said <PERSON>, 'and what say you now?'
'Alas!' said <PERSON>. 'I will not say that she is the fairest lady that lives.'
'Then I must go for my axe,' said <PERSON>. 381