For you to be jealous, you must believe that their "lot in life" is better than yours. In making such a statement, you define a way to compare lives: "Life X" is better than "Life Y." You create a what mathematicians call a poset of lives: a set of lives with comparison operations like < and => between them.
From your original question, you measure "life in the future" as better than "life in the present."
As for justification, consider that mappings such as this are inherently without meaning. With no extra effort on your own, you can define a mapping where "Life in the present" is better than "life in the future." Defining them is easy.
What seems to be important is that you find this particular mapping of yours acts on you. You see the world differently because of it. You act differently because of it.
Dispose of the question of whether your mapping is "right" or "wrong." Such wordings are so difficult to work with that religions have literally fought wars over disagreements and failed to come to an accord. Instead, you may be able to justify your stance by looking at your actions.
Most philosophers will generally agree that "doing good things" is morally good. There will be hundreds of squabbles over whether it is perfectly true, but in this case I think a minimal agreement that those word are generally "right" is all that is needed.
Look at how you are, and how you act. Now picture how you would be and how you would act if you did not hold this jealousy. If you find this jealousy is a positive force in your life then you may be able to "justify" it. If your jealousy drives you to learn more and meet more people and generally be a "good" person, then it will be hard to argue against it (not impossible... there are arguments, but that's okay for now... you're looking for a justification, not a proof of righteousness).
On the other hand, if you find your jealousy is a negative force in your life, then you will have trouble "justifying" it. If you find your jealousy is driving your friends away, making you depressed, or making you into a "bad" person, then justification will be hard to come by because the evidence stands against it.
From your wording, you are seeing those negatives, and you are worried about them. This is good: it means you are being self-aware and not taking yourself for granted. I am not in a position to judge whether the positives outweigh the negatives. You must do so yourself. Remember that there are likely positives to your jealousy. Few hard-to-kick behaviors don't have some positives; they just have more negatives. Its your job to figure out whether the positive outweigh the negative or vice versa.
I will note that most religions have found jealousy to be a trait where the negatives outweigh the positives. I do not argue the truthfulness of their statement, merely point out that there is a strong statistical precedent for you to consider. (And an amusing consequence of your stance: you should appreciate the wisdom brought to you previous generations. You live in their future, and get to take advantage of their hard work in the past ;) )
One thing which I do believe would be good for you is to try to understand your friend's position. You don't have to agree, but just understand. His stance is along the lines of a very classic one: he has found that there are no mappings life "life X is better than life Y" which benefit him besides "I want my life more than I want anyone else's life." He rejects those other mappings and simply acts without them.
The reason I recommend understanding your friend's position is because it has been found to be a very useful position through history. Forces like jealousy drive you forward because they all drive together into a powerful force: "I want something." If you want something, you can aspire towards it. "Want" is powerful, with immense positives and negatives. It drives much of society. You will craft your "wants" throughout your life.
Your friend's position is one of "no want." He isn't looking to change, he isn't aspiring to something (at least his words say so... leave it to him to decide if he is actually aspiring or not). "No want" is also powerful, with immense positives and negatives.
If you understand both of them, you can use both at your leisure. While your jealousy does you good, you keep it. When it stops working for you (and many religious philosophers would say it surly will), you can discard it because you understand the other side of the coin. When "no want" fails you (and much of Western society would say it surly will), you can pick up wants, because you are now aware of both options.
Taking both of these positions together, you have a third option: balance. I would dare to speak for others and say that a small amount of jealousy is not unreasonable. A small amount of jealousy for the future, rather than a crippling jealousy for the future might actually provide the most positive outcome. With so many vices and complexes, the "benefits" appear in small doses, but the negatives pile on as the vice or complex grows.
A little of what you like does you such a lot of good.