In addition to the excellent points above about not being able to offer reassurance because it would be false reassurance, many Ts won't offer reassurance because of the belief that it offers short-term relief but actually reinforces anxiety in the long run. The theory as I understand it is that anxiety has a cycle. If left alone, it will rise and then peak and subside. Any attempts to avoid the anxiety (like reassurance-seeking) interrupt that cycle instead of allowing it to peak naturally and subside. And there is relief in that, but the theory is that it reinforces the notion that you were right to be anxious in the first place by sending the message that whatever you were anxious about is indeed dangerous and that action must be taken to avoid it. And so, by taking action to avoid your anxiety, you inadvertently strengthen/reinforce it. I don't know if there is solid research to support this notion, but I think it's a theory that a lot of therapists subscribe to.