Joining the military means giving up most of your civilian rights laid out in the Bill of Rights, in exchange for pay and training. Historically, this has often been a pretty good deal for those without other economic options.
However, if you think the country's leaders are going to send you off to get killed in poorly thought-out and reckless adventures for dubious reasons, then giving up those rights starts looking less and less attractive. Given that's what happened in Afghanistan (which was somewhat justifiable, but could have been a six-month operation, rather than a 20-year one) and Iraq (completely unjustifiable, an epic disaster), it's not surprising that economic desperation is now the main motivator for military recruiting, nor is it surprising that applicants coming from that background aren't in the best mental or physical health.
Step one in regaining trust would involve purging the government and military leadership of the architects of the debacles in Iraq and Afghanistan, although that seems unlikely at present - these are the same neocon ideologues who are vigorously opposing a negotiated settlement in eastern Ukraine while also pushing for military confrontation with China.