This rhetorical question is a typical one lobbed at atheists when atheist backs the theist into a corner about proving the existence of their god. While its function is rhetorical, the theist has actually committed an analogical error when throwing this question back at the atheist.
As the “arguments” go, the atheist will charge the theist with demonstrating the existence of their god. After exhausting the usual arguments for the existence of their god, they'll fall back on the statement “I can't prove that [my] god exists
. Can you prove that your mother loves you?” The reason that this is an analogical error is because, to use an overused phrase, they're comparing apples to oranges.
Let's break it down into a more variable format: “I can't prove X exists”. For the theist's rhetorical “gotcha” to make analogical sense, they should replace the X (initially “god”) with some other noun. Now it's obvious that this no longer has the same rhetorical power that saying “can you prove your mother loves you?” has. In other words, the more proper analogy would be “I can't prove that [my] god exists. Can you prove your mother exists?”. If anything, the contrast to “can you prove your mother loves you?” would be “can you prove that god loves you?” (obviously, the thing that we are claiming that does the love has to be proven to exist in the first place).
More explicitly, the theist is conveniently replacing the ontological status of some noun (i.e. god) with the subjective experience of the effect of some verb (i.e. someone loves you).
The theist's rhetoric at this point seems to be meant as a cognitive stop sign; using emotional manipulation to stifle the argument. Not because they're trying to make an analogical point. The “logic” is that since I can't prove that my mother loves me, yet I still continue to believe that she does, it is equivalent to the theist's continued belief in their god's existence even though they can't prove it. No. The equivalent of the theist maintaining their belief in their god's existence even though they can't prove it would be to continue to believe in [some other noun besides god]'s existence even though they can't prove it. When you insert any other noun besides “god” in that variable, the analogy is becomes more damaging to the theist's case.
Basically, if a theist throws this rhetorical question at an atheist, they might as well concede that they've lost that particular debate.