Why does one act agains themself?

because as people we feel safer settling for things vs going out of our comfort zone to fix them
like putting off homework because you procrastinate
or staying in a situationship because you like the attention

Because of laziness in my opinion. Every improvement needs effort but people are too lazy to make that change happen. This can be seen in people smoking for example, people know that smoking is bad and harmful but they never stop because they are too lazy to put in any effort to change.

In most cases option A will be the better option and will take more effort and more capacity. Option B is worse but will take less capacity and less effort and thus make it a more comfortable option. :/

usually, if someone is doing something that is harmful to themselves, it's because they get some kind of benefit from it in some way, whether that benefit is real or just imagined, and/or because the negative effects of doing something harmful are so far off that they're imperceptible. smoking is a good example; nicotine makes you feel good, it relieves stress. it might kill you in the long run and take years off of the end of your life, but in the here and now, an 8, 10, 12, 14 hour workday takes all these hours off of your life right now, so a little bit of cancerous de-stressing is worth it for many people. same thing with alcohol, or any other drug. 

many homeless people are addicted to drugs not because they're inherently weak or bad people, but because being homeless is incredibly stressful in ways you could not even imagine. in the US, you can't get a job if you don't have an address or a bank account and homeless shelters are often full or have waiting lists, or treat the people who need that resource like garbage. all of these are just some of the things directly preventing homeless people from working their ways out of that rut. i'd probably be smoking crack too if that was the only way to make anything better. 

if someone stays in an abusive relationship, it's probably because they need that person in some way, or they believe they do. for example, a woman may have become financially dependent on her abusive husband so she can care for their children, and she may not be able to get a job in order to escape him because she's spent the last 10 years caring for the kids at home instead of building up a resume. she has no work experience, she can't easily get a job good enough to pay for the 2, 3, 4 kids on her own. this goes double if he's powerful in some way, such as being a cop, triple if the woman is disabled. even without kids, these situations happen, and it can happen to men, too.

even simple things, such as a kid not getting their homework done, has pretty easy explanations like this. maybe they don't feel like they have enough time in the day to get what they want to do done, so they procrastinate. maybe they don't understand the material and feel it's better not to try at all than it is to try and stress themself out with material they don't get. maybe they struggle to stay focused due to something like ADHD, and are constantly fighting themself to get things done because they either need medication, or an alternative approach to just sitting down at a desk for 4 hours straight.

people don't do "A" because to them, "B"--the more harmful approach--has some kind of benefit that they wouldn't be getting from "A". so if you see someone struggling with something like this, don't constantly try to preach how "A" would be so much better for them. they are probably aware of how much "A" would be better for them, but they have reasons for not working with it, ones that usually aren't just them being 'lazy'. you should create some kind of "C" instead--compromise--that gives them the benefits of "B", or something that could replace what they're getting from "B", while still allowing them to also have the benefits of "A". this is a more personalized method formulated to the person's specific needs, so they can work around their issues in healthier way that works for them.

for example: if quitting smoking isn't feasible for the person right now due to how stressed they are, and
smoking or any other kind of drug to replace smoking is harmful, then
you need a compromise, like nicotine gum.

if someone isn't choosing "A", it's because "A" doesn't give them what they need, it won't work for them. just because "A" is what you think someone else should do doesn't mean it's a good option for them right now, and wasting your time only trying to force "A" on someone else is only going to push them away from you in the long run. if "A" doesn't work, and "B" sucks because it's harming them, look for "C"--compromise.