According to my own reading and Wikipedia:
On the basis of their language, content, and other factors, the Pastoral Epistles are today widely regarded as not having been written by the Apostle Paul, but after his death. (Although the Second Epistle to Timothy is sometimes thought to be more likely than the other two to have been written by Paul.) Critics examining the texts fail to find their vocabulary and literary style similar to Paul's unquestionably authentic letters, fail to fit the life situation of Paul in the epistles into Paul's reconstructed biography, and identify principles of the emerged Christian church rather than those of the apostolic generation.
One of the early critiques of the letters point to the large number of hapax legomenon found in them compared to the rest of Pauline corpus. But it seems that analysis has been shown to be flawed.
More difficult to me are passages like:
(No longer drink only water, but use a little wine for the sake of your stomach and your frequent ailments.)—1st Timothy 5:23 (ESV)
O Timothy, guard the deposit entrusted to you. Avoid the irreverent babble and contradictions of what is falsely called “knowledge,” for by professing it some have swerved from the faith. Grace be with you.—1st Timothy 6:20-21 (ESV)
When I send Artemas or Tychicus to you, do your best to come to me at Nicopolis, for I have decided to spend the winter there. Do your best to speed Zenas the lawyer and Apollos on their way; see that they lack nothing. And let our people learn to devote themselves to good works, so as to help cases of urgent need, and not be unfruitful. All who are with me send greetings to you. Greet those who love us in the faith. Grace be with you all.—Titus 3:12-15 (ESV)
Quite personal and specific instructions are sprinkled throughout the Pastorals, which is one of the very reasons they are called "pastoral". So under what scenario could these fairly early Christian texts be accepted as authentically from Paul if they were written by someone else?