Some people are able to afford the posh "backpacking" that is traveling around Europe - which is a custom with a lot of people that have considerably more money than I do. Therefore what I am going to say here does not apply to that and I actually have no experience with doing any of that sort of backpacking at all.
I do feel though that for a majority of backpackers, whether it is a gap-year thing or just a prolonged vacation later in life, that they are likely going to head to other cheaper places such as India, South-East Asia, or Central America.
There is one thing that all of these places have in common outside of rare elevated locations: They are extremely hot.
I grew up in the 80's and 90's so for me getting outside during spring and summer was always something that never really affected me a great deal, at least that I can remember. This could be the endless energy of youth or the fact that we normally combined this with some sort of swimming but the real fact of the matter is that the place where most people in the West grow up can't even come close to the levels of extreme heat and humidity that one will experience as soon as they leave the airport in their starter country.
For me, I remember the immediate and intense sensation that I felt as soon as I walked out the front door of the Singapore airport: It was like a wall of sweltering suffering hit me straight away. I was foolishly still wearing my airport clothes and soon found myself dripping just waiting in the taxi queue to get to my hostel.
I immediately became self-conscious of the fact that I had already sweat my shirt through to the point where it looked like I had jumped in one of the fountains before getting in line and of course, I was extremely uncomfortable. By the time I got to the hostel, I had to sit in front of a fan for 20 minutes or so just to recover before I even checked in. The receptionist got a laugh out of this but she was being good-natured about it and even asked me "first time to Singapore?" because she had seen this situation many many times in the past.
It was at this point that I realized that a vast majority of the things that I had brought with me were completely inappropriate clothes for the areas I had intended to travel to. Singapore is very near the equator, which means that it is one of the hottest places on the planet. Surprisingly, I noticed that very few of the locals were sweating at all but I suppose this all boils down to genetics. Even though Singapore is a mish-mash of nationalities, you can easily identify people that could be considered natives because they are the only ones not sweating buckets everywhere that they go.
I used to kind of poke fun at people that would make a distinction between heat and humidity, but let me tell you, it makes a word of difference. Where I had grown up in the states, the temperature would routinely top 100 degrees F (38 C) but we were also experiencing a dry heat which meant that we were around 20 percent humidity. In Singapore, almost all of the rest of SEA and also similar countries in Central America this jumps up to around 90 percent. It makes a world of difference.
I have always been someone who sweats a lot despite being relatively active most of my life, but this meant that I was going to be uncomfortable just about all the time and a lot of this had to do with the clothing that I chose to bring with me.
There are a few things that you can do to beat the heat and these again are rookie mistakes that I didn't know any better because it was never mentioned to me and it really should be.
For one thing, go ahead and forget about all of your favorite cotton t-shirts that you got in your home country: They are useless and will make you feel like you are wearing a sweater at all times. They are designed for the climate that you have in your home country and serve as intense insulators that are going to make you suffer if you are in a tropical country.
One choice that people make that I feel is disrespectful and a mistake is to immediately switch over to a sleeveless vest / singlet / wife-beater shirt. Obviously this is going to make you feel cooler but it also is quite inappropriate in many locations specifically in South East Asia where the population is quite conservative. You will get funny looks wherever you go if you are dressed like this (and not at the beach) and especially if you brought it from "home" there is a good chance it is made of the same thick cotton that your oppressive T-shirts are made of anyway.
I have said many times in the past to avoid packing as much stuff as possible but I want to say it again that this is particularly true with almost all T-shirts that are made in the West. They are simply far too heavy for the climate in all of the tropical world and are therefore just going to sit in the bottom of your backpack until you eventually throw them out / give them away.
The best thing to do is just to find the lightest material that you can, maybe a workout shirt or two with the "Clima-cool" magic fabric and bring 3 of those. Then just buy all the rest of your shirts while you are there since they cost next to nothing at all the locations I have been in any inexpensive tropical country.
If you think there is a day where you are going to need to dress up to go to a club or nice restaurant I am going to go ahead and tell you that this is incredibly unlikely to happen and even if you did meet the woman / man of your dreams and this situation did arise guess what? They have shopping centers all over the world and you can get them there for probably a lot less than they cost in your home country.
So when you are looking at your T-shirts when deciding which ones "define you as a person" in your packing process. Take less time focusing on what the shirt looks like and more time on what it is made of. If you hold it up to your mouth and can not breathe through it, don't take it with you because you will later regret it.