Some clients take their lawyer's opinion about their case personally. When they hear something negative about their case, they start thinking that their attorney is either incompetent or insecure or is unwilling to fight for them. While in some (probably very few) cases it's true that the attorney has improper motives, usually there are legitimate reasons as to why your lawyer would share this kind of information with you - it's because that lawyer has real concerns about your case and he wants you to have realistic expectations about your case and understand the different issues that pretty much every employment case has and many other kinds of cases have.
You might think that your attorney's job is to fight for you and your case. While this is in part true, you also have to remember that your lawyer's equally importnat job is also advise you about the merits of your case and make sure that the fight you are both fighting is fought most correctly and most wisely in light of the unique facts of your case.
An attorney who is not sure about your case is probably more trustworthy than someone who is overconfident and who simply tells you what you want to hear.