1. I just don’t have the mental capacity and energy to devote in order to respond appropriately. I’d rather ignore it until I’m in a better place to answer than send a thumbs up or something.
2. I sometimes don’t know how to respond. I like to keep conversations going, so I really dislike sending back one word answers. But sometimes the texter doesn’t really leave me with many options, so I sit on it and then forget.
3. I’m fucking busy, man. Non-urgent messages don’t take priority. That doesn’t mean you are not a priority, but in that moment you are represented only by some words on a screen that have nothing to do with what I’m currently focusing on.
4. Because I’m tired of being a landfill for their problems. They only come to me to vent about their troubles and any attempts to steer the conversation away from that leads to a dead end. I’m not a therapist. If all they got for me is a tiring conversation that’s gonna leave me feeling drained, it’s not worth my time.
5. Because my mood is already crappy from some other unrelated occurrence. I know I will be somewhat rude once I’m in a crappy mood, and I would prefer not to subject my family to my mood swings.
6. I’m in a crappy mood and don’t want to be rude.
7. It’s something I don’t know how to respond to or I just find it pointless to reply to.
8. My family is fairly large and very, umm, spirited in expressing strongly held opinions. They also are less than enthusiastic about viewpoints that disagree with them.
Do I love my family? Absolutely 100%! Would I give any of them one of my kidneys to save a life? Without hesitation! Do I get really fucking tired of hearing about why the moon landing was fake and sometimes just ignore the message? Yep.
9. Depression type situation usually. I do this to my brother all the time. Love him to death but he is about the only one I don’t think would be too upset to not get an immediate response. It’s nothing personal at all, it’s like I can’t even with life right now.
10. Sometimes I just don’t feel like talking to anyone so I ignore them…
11. Because I don’t feel like it. Just because anyone can instantaneously demand my attention doesn’t mean I owe it to them.
12. Social interactions can be exhausting.
13. I hate how I have this device with me 24/7 that gives everyone I know the ability to stop what I’m doing and turn my attention to them.
14. They’re being needy or annoying (ex: asking for water while they’re perfectly capable of bringing it themselves).
15.Because sometimes it’s something completely meaningless and I don’t think I actually have to answer.
16. That’s the best part of text messages: plausible deniability. You can pretend you didn’t see it for awhile, and no one can prove you didn’t. I use that all the time.
17. I occasionally “ignore” them for a couple of hours because I decide for myself when I want to have a conversation and when I want to focus on what I am doing right now.
18. People will call if they need to get hold of you urgently. Text messages are not urgent, under any circumstances. Ever. Either call me, or accept that I will read my messages when at a time that suits me. And that may take a while…
19. Too anxious to reply.
20. Sometimes as I can’t think of a reply, then time goes by and it seems awkward to then send a reply with a “sorry it has taken me three days to respond” half arsed message in it. So I bury my head in the sand.
21. Because of depression. I just don’t want to do anything. Don’t want to talk, be social, etc. It’s too mentally and emotionally taxing, and I gotta spend what little will that I do have on stuff like eating, showering, and getting dressed and whatnot.
22. If it’s a ‘life story’ size message I have to be in the mood to read it and put effort into replying, I know that sounds bad but I want to reply with equal enthusiasm and if I’m not in the mood or my mind is on something else my replies can often be flat and it ruins conversations. However if it’s something that requires an urgent response I will respond within the hour.
23. Because I’m fucking exhausted and I can’t stand even an ounce of human interaction right now.
24. Sometimes life is overwhelming and being able to ignore a few messages feels like a mental break.
25. I feel like the phone is a “cheat”, so I try to stay away as much as I can without being a total hermit.
26. I get distracted while deciding what response to go with and end up not texting back.
27. I open it and TRULY forget to respond and then proceed to forget I ever got the text in the first place.
28. I’m pretty selective with whom I message.
29. Sometimes I just want to enjoy my personal zone and not talk to anyone, even though I often really love the people I “ignore”.
30. Sometimes I just don’t have the energy to converse with someone. I just don’t feel like maintaining a conversation at that moment. 
Source: https://thoughtcatalog.com/january-nelson/2020/08/30-reasons-why-people-ignore-texts-from-people-they-love/