When I heard the news that Juan Carlos was going to abdicate. I thought well if he abdicates maybe The Queen will be a little bit tempted. I still don't think she will abdicate I am just saying it might just cross her mind that its ok to abdicate.
I personally think Monarchs should reign for life by the way.
The Spanish vs. British circumstances are very different.
JC is under a lot of public scrutiny both because of his own actions and his daughter and son-in-law's actions. He is visibly suffering in health. Support for the monarchy in general and JC in specific has been plummeting in recent years. There was already a strong republican movement in Spain, which is only bolstered by the various separatist movements.
JC's decision to abdicate can be seen as something that he's done to help save the Spanish monarchy. He should be applauded for his role in transitioning Spain to democracy, but now in many ways he's outlived his usefulness and if he were to continue to cling to power he would risk the downfall of the monarchy. While republican beliefs are strong in Spain, there is also a lot of support for Felipe, so it's possible that under his reign he could improve the image and standing of the institution.
EII is not under public scrutiny for her actions, and is frequently one of the most popular royals in Britain. Whatever health problem she has (and at 88, she surely has some), they've been kept private. There isn't nearly as strong of a republican movement in Britain, and while there are separatists movements (particularly in Scotland) they're also linked to a debate about achieving independence while maintaining a connection with the crown, like the other Commonwealth Realms. Further, the existence of the Realms makes it that much harder for EII to abdicate - each one of her parliaments would have to pass legislature to allow her to do so (while only Britain would have to act if a Regency were established).
Further, EII abdicating could actually have serious repercussions on the institution of monarchy in Britain. Since the 90s there has been a push for Charles to abdicate in favour of William - fueled by a combination of comments made during the War of the Waleses and a common misunderstanding of the circumstances under which Edward VIII abdicated. While Charles' popularity is increasing and the demands that he abdicate in favour of his son are decreasing, if EII abdicated then it could reignite this - if she can abdicate then why can't Charles? And as such it could very easily turn the British monarchy into a popularity contest. Not to mention the fact that regardless of where people stand on the issue of the Cambridges' duties to date, I think few would argue that William and Catherine are at this point in time ready to b King and Queen.
JC abdicating has the potential of saving his realm, at least for the time, while EII abdicating would have the potential of destroying her realms.