When The Daily Mail published parts of a private letter that Meghan Markle had written to her father:?

Totally agree with you!

She seems to think she has rights but no responsibility ........ is that because of her upbringing in the US culture, where rights are alway promoted and responsibilty is ignored , where sue culture is 'normalised' anyway or another US cultural thing is a total misunderstanding of Royals where they tend to think  of Royals in terms Disney princess stories which is totally unrealistic , is it an 'age thing' of entitlement, is this a mental health issue being a product of divorce parents where many children assert blame and take sides, joining with Harry who mentally is stuck aged 12 yrs old and clearly states " he didn't want history repeating itself" when his mothers death was an accident caused by a drunk driver, a lady who courted the press when it suited her to do so and she was living a 'wild child' lifestyle of high risk/high reward, but Harry blamed the press for her death then and is completely stuck in that 12yr old mindset and a parent who dies becomes a god/goddess who can do no wrong.

The two of them together with back history like they have support and encourage each others mindset   .... I don't know could potentially be partly all of them

There are privacy issues in either case.  Where there is a written letter, there is also a copyright case, as the republication without her consent would have to fit in some legal exemption (such as "fair use for news reporting" or the like).  Just because someone possesses a copy of your original work doesn't give them any legal license to duplicate an distribute it.

Same for things you post online: you typically only license the host to distribute it on its site, not for anyone ELSE to download a copy and repost it somewhere else for their own advantage without your permission.  You can sue them, at least under US copyright laws.

I think she wanted that info leaked to the press. She does not know when to shut up.

She didn't win when she sued the Mail though did she, because her father gave the letter willingly.  

She sued because her private personal letter was published without consent. 

Leaking the contents of a call between Harry & William isn't her call, her privacy, her issue. It's up to W&H to take action if that's what they want to do.

No, the phone call was between two other people so she has no legal interest thus she cannot sue.

Hope this helps

Good question, but only Meghan can answer that.