They stick there like glue!

(Thomas Boston, "Human Nature in its Fourfold State")

The memory bears evident marks of sin and corruption.
What is good and worthy to be remembered, makes but
slender impression, so that impression easily wears off; the
memory, as a leaking vessel, lets it slip. As a sieve that is
full when in the water, lets all go when it is taken out—just
so is the memory with respect to spiritual things.

But how does the memory retain what ought to be forgotten!
Sinful things so bear in themselves upon it, that though men
would sincerely have them out of mind—yet they stick there
like glue! However forgetful men are in other things, it is
hard to forget an injury. So the memory often furnishes new
fuel to old lusts; makes men in old age remember the sins of
their youth, while it presents them again to the mind with
delight, which thereupon returns to its former lusts.

Thus the memory is like a riddle—which lets through the
pure grain, and keeps the refuse.