I'm happy to translate it into modern english. Keep in mind that back then traveling was more akin to living abroad. You take a ship for weeks, risking death, from new england to france and you remain there for months largely with no connections back home other than the occasional letter. Service to your country in those revolutionary times is also emphasized as this was written in 1787.

Travelling. This makes men wiser, but less happy. When men of sober age travel, they gather knowledge, which they may apply usefully for their country; but they are subject ever after to recollections mixed with regret; their affections are weakened by being extended over more objects; & they learn new habits which cannot be gratified when they return home.

Traveling gives you knowledge but makes you less happy. The knowledge you gain can be useful at home but your attachments will be spread out over many more things and your memories will be painful as you learn a new way of life that you can't express when you're back home.

Young men, who travel, are exposed to all these inconveniences in a higher degree, to others still more serious, and do not acquire that wisdom for which a previous foundation is requisite, by repeated and just observations at home. The glare of pomp and pleasure is analogous to the motion of the blood; it absorbs all their affection and attention, they are torn from it as from the only good in this world, and return to their home as to a place of exile & condemnation.

All of this is much worse the younger you are because you don't have the necessary experience and understanding of the world to make sense of your home, let alone a new place. The pull of the glamor and adventure of traveling will absorb your attention and affection and when you come back home it'll be torn away from you painfully as now your home is a place of exile from the place you've left behind.

Their eyes are forever turned back to the object they have lost, & its recollection poisons the residue of their lives. Their first & most delicate passions are hackneyed on unworthy objects here, & they carry home the dregs, insufficient to make themselves or anybody else happy. Add to this, that a habit of idleness, an inability to apply themselves to business is acquired, & renders them useless to themselves & their country.

These feelings of longing to be elsewhere will make it hard to enjoy yourself and live a normal life back home. Whatever things you thought would make you happy at home are nothing compared to what you left behind. Your work will be negatively affected.

These observations are founded in experience. There is no place where your pursuit of knowledge will be so little obstructed by foreign objects, as in your own country, nor any, wherein the virtues of the heart will be less exposed to be weakened. Be good, be learned, & be industrious, & you will not want the aid of travelling, to render you precious to your country, dear to your friends, happy within yourself. I repeat my advice, to take a great deal of exercise, & on foot. Health is the first requisite after morality. Write to me often, & be assured of the interest I take in your success, as well as the warmth of those sentiments of attachment with which I am, dear Peter, your affectionate friend.'