to be filled by my vanity, my love, my hope, my body. (3-5)
Ah, this is where it all begins. The suit, which has been waiting on the chair while the speaker showers, is ready to be filled with the speaker's vanity, love, hope, and body. Whew, that's a lot of stuff to fit into one suit. It's no wonder that the speaker changes the suit from merely fabric into something resembling himself when he puts it on. We just don't know what it'll transform into (not yet, anyway).
my legs seek the hollow of your legs, and thus embraced by your untiring loyalty (10-13)
Using personification, Neruda lets us see that the suit is already becoming more human. It has its own legs, though they are hollow, but referring to them as "legs" doesn't necessarily mean we think they are alive. It's the use of "your" that reminds us that Neruda is addressing the suit. They then "embrace." The suit has transformed into something that can return affection. It's more than just a suit (after all, how can a piece of clothing be "loyal")? The speaker is beginning to consider the value of the suit; it's transforming into something more than just a nice cut of fabric.
I am shaping you, poking out your elbows, (27-28)
The speaker compares the "shaping" that the world does on him to how he shapes the suit. The more he wears it, the more it wears out. But perhaps, it also becomes more comfortable, more fitted to his body. This transformation mirrors the transformation he is going through in his own life, as events and struggles wear him down and "shape" him. Life wears him and transforms him, just as he wears and transforms his suit.
and so your life grows in the image of my own. (30-31)
The word "grow" conjures up very specific imagery. It makes the reader picture grass, and trees, and maybe even cute fuzzy animals learning to walk. Growth is a type of transformation; it's a very natural way to transform. Here, Neruda shows us that whatever is changing, it's happening naturally.
because we are one being and shall be always (65-66)
At the end of the poem, the speaker ponders the end of his life. As he imagines dying, he realizes that he and his suit are "one" and always will be, even unto death. The suit has gone from a pile of clothing sitting on a chair to a part of his existence, though in these lines the speaker realizes that it isn't the suit that has transformed; the real transformation happened in his mind the moment he decided to take a second look at something old and familiar.