I finished my Ph.D. recently and I am about to submit my thesis but I have copied a few lines from the discussion and results from my own papers. Will it be considered plagiarism? Can I exclude my papers while running Ithenticate software or should I contact springer nature and ask for permission? Please let me know if I need to quote text copied from my paper.
I quote Springer's policy about how author should reuses their own articles in their thesis. You may check it yourself.
"Authors have the right to reuse their article’s Version of Record, in whole or in part, in their own thesis. Additionally, they may reproduce and make available their thesis, including Springer Nature content, as required by their awarding academic institution. Authors must properly cite the published article in their thesis according to current citation standards. Material from: 'AUTHOR, TITLE, JOURNAL TITLE, published [YEAR], [publisher - as it appears on our copyright page]’ "
Apparently the copyright of the earlier work is held by Springer (or another). That implies that you need to treat the work like any other copyrighted work, though you hold a certain license from them for certain uses (typical case). You need to abide by that license.
You would be best to quote and cite the earlier work, which I can't see as being a problem in a dissertation. Straight up copying with no indication that it is copied could be seen by those judging your thesis as improper self plagiarism.
Note that copyright and plagiarism are distinct. The former is governed by (usually civil) law and the latter usually only by custom (with some exceptions unlikely to apply to self plagiarism).
The license from the copyright holder is probably generous enough that you can quote and cite longer passages than you could from the work of another but that doesn't solve the (self)plagiarism issue.
Note that self plagiarism is an issue because reader of a scholarly text wants the complete context of the ideas to be readily available. Some of that context is only in the earlier papers, so you want to keep in clean and clear.
Caveat: I'm assuming that the "few lines" contain significant information pertinent to the conclusions. A few lines of "filler" not essential to the whole is probably inconsequential.
It is technically plagiarism but not of the type that will get you in serious trouble. They are your words, after all. But you can very simply mitigate this - just quote yourself and cite the paper. Or, rephrase it. The rule is in place to prevent one from appropriating others’ work as their own