Love sonnets of Drayton's age usually pay compliments, and this one starts as if it will. "To nothing fitter can I thee compare" sounds like the opening of a flattering list of roses and stars. Instead it lands on "the son of some rich penny-father." The person addressed is likened to a young heir who squanders his dead father's savings, which is a very odd way to begin a poem of affection.
The first eight lines tell this little story, and the word choices do much of the work. "Penny-father" is a scornful compound for a man who counted every coin. He has "brought on his end with care," and "care" works two ways: thrift, and the worry that wore him out. What he leaves is something "heap'd together," a pile gathered without joy. His son is a "novice" at wealth, "lavish of his chest," and the syntax scatters as wildly as the money. He "to one man gives, doth on another spend, / Then here he riots," and the clauses hop from recipient to recipient without settling. Then the line steadies: "Haps to lend some to one true honest friend." The verb "haps" is almost an accident, so the one reliable person gets his share only by luck. The four plain words "one true honest friend" slow the rhythm after all the commotion.
At line 9 the poem turns and the story becomes an accusation. The money vocabulary carries over, but now the currency is human: "gifts," "kindness," and "love" are all being "waste[d]" or badly invested. "Thy love that is on the unworthy plac'd" pushes "unworthy" forward by inverting the normal word order, so the reader trips over the very people the poem disapproves of. The feminine rhyme of "deceive thee" and "leave thee" links two fates, being cheated and being abandoned, as if the false friends and passing time were the same thief. Time is the last and surest spender here. Beauty, usually the subject of praise in a sonnet, is treated as a shrinking inheritance: "Time hath thy beauty, which with age will leave thee." The tone is closer to a friend's warning than a lover's worship.
The form helps. This is the English sonnet pattern, abab cdcd efef gg, in mostly regular iambic pentameter. Drayton lets the rhyme-sounds enforce the sense. Several lines end on an extra unstressed syllable ("father," "together," "deceive thee," "leave thee"), which gives them a slightly trailing, conversational feel, as though the speaker is talking rather than declaiming. The pattern of the argument also stands out. Eight lines of story are followed by four of application, then a couplet that turns the whole comparison back on the speaker.
That couplet is the poem's best move: "Only that little which to me was lent / I give thee back, when all the rest is spent." We realise the "one true honest friend" of line 8 was the speaker all along, and the simile was a quiet piece of self-portraiture. The chiming of "lent" with "spent" is pointed, because his loyalty arrives only after the ruin. "That little" can sound modest, or it can sound stung. His love is treated as a loan that he now returns, and this can be read as a promise to be there when everyone else has gone, or as a cool closing of accounts. The poem doesn't settle which, and the uncertainty makes the ending feel truthful rather than neat.
The poem matters because it takes a form built for idealising and uses it for plain speaking. Instead of describing the beloved's perfections, it gives advice, with an everyday image of money and inheritance that any reader can follow. It keeps the tenderness and the reproach in the same breath, and it gives the speaker the least glamorous role available, that of the friend who lends a little and is still there at the end. The final two lines suggest that this steady, unspectacular kind of love may be worth more than all the spent fortune.