This manga panel has much more narrative significance than one may initially believe. Not only does it give an insight to their desire for heroism and the need to prove themselves, but it also gives a hint to an undisclosed side to Eren that we may have not known about, a thread.

Eren is a great character, but he is also petty and selfish. This parallel to Reiner might imply that Eren too must've enjoyed sniffing Historia's letters. The reason why Eren told Historia of his plan but not others is not out of companionship, but to sniff more of her letters.

The reason why Eren blamed Reiner for his mother's death, for which he is at fault, is just him projecting his own insecurities and venting his jealousy, as Reiner gets to sniff more post-rumbling, onto Reiner. End of thread.

I hope this may help you see Eren in a different light and understand how nuanced and dedicated sniffer of a character he is. Thank you for reading.