It expresses the changing in a duration. The beginning and end of this duration are indefinite. It is a common suffix.

This is not "le+ş(reciprocal/together)". "-leş" is entirely different suffix.

This is a bit irregular topic. You can´t bring this suffix to every verbs even if it is really very common suffix, it is used very much.

güzel (beautiful)

güzel+leş+mek

güzelleşmek (to become beautiful)

Görmeyeli çok güzelleşmişsin. (You have become beautiful very much since I/we haven´t seen.)

iyi (good)

iyi+leş+mek

iyileşmek (to get better)

Hastaydım ama iyileştim. Şimdi iyiyim. (I was sick but I got better. I´m fine now.)

zor (difficult)

güç (1the force, the power, 2difficult)

zor+laş+mak, güç+leş+mek

zorlaşmak, güçleşmek (to get difficult)

Yağmur hızlanınca yürümek zorlaşır. (It gets difficult to walk when the rain speeds up.)

kötü (bad)

kötü+leş+mek

kötüleşmek (to get worse)

Ekonomi son aylarda iyice kötüleşiyor. (The economy has been getting worse in recent months.)

bir (1one)

bir+leş+mek

birleşmek (to unite (intransitive))

Birleşmiş Milletler (the United Nations (direct: the nations that have united))

ağır (1heavy, 2slow, 3difficult)

ağır+laş+mak

ağırlaşmak (1to get heavier, 2to become heavy, 3to get difficult)

Yağmur yağdığı için pantolonum da ıslanıyor ve bu yüzden ağırlaşıyor. (My trousers are getting wet because it is raining and therefore it is getting heavier.)

"ağırlaşmak" can´t be used instead of "şişmanlamak (to get fatter)".

Sorular ağırlaştığı için sıkılmaya başlarlar. (They start to be bored because the questions get difficult.)

kalabalık (1the crowd, 2crowded)

kalabalık+laş+mak

kalabalıklaşmak (to become crowded)

Cadde genellikle akşam sekiz buçuktan sonra kalabalıklaşıyor. (The street is usually becoming crowded after eight o´clock in the morning.)

kesin (certain)

kesin+leş+mek

kesinleşmek (to become definite)

Sınav tarihi kesinleşirse size haber vereceğiz. (We will inform you if the date of exam becomes definite.)

aptal (stupid, fool)

aptal+laş+mak

aptallaşmak (to become fool)

Küçükken çok zekiydi ama sonra aptallaştı. (S/he was so clever when s/he was little, but s/he became fool later.)

karmaşık (complicated)

karmaşık+laş+mak

karmaşıklaşmak (to become complicated)

Olayları karmaşıklaşmadan düzene sokmamız gerek. (We need to put the events in an order before they become complicated.)

beyaz (white)

beyaz+laş+mak

beyazlaşmak (to become white)

Yedi senelik gömlek. Griydi. Beyazlaştı. (It is a shirt for seven years. It was grey. It became white.)

kırmızı (red)

kırmızı+laş+mak

kırmızılaşmak (to become red)

and other colours as well

şehir (the city)

şehir+leş+mek

şehirleşmek (to become city)

Burası çok küçük bir kasabaydı, köy gibiydi, ama birkaç yıldır şehirleşmekte. (This place was a very small town, it was like a village, but it has been becoming city for a few years.)

zengin (rich)

zengin+leş+mek

zenginleşmek (to become rich)

"Türkiye zenginleşiyor." diyorlar ama hâlâ bir sürü işsiz insan var. (They are saying "Turkey has been becoming rich." but there are still many unemployed people.)

fakir, yoksul (poor)

fakir+leş+mek, yoksul+laş+mak

fakirleşmek, yoksullaşmak (to become poor)

çağdaş, modern (modern)

çağdaş+laş+mak, modern+leş+mek

çağdaşlaşmak, modernleşmek (to become modern)

Arap ülkeleri hâlâ tamamen çağdaşlaşamadı. Afrika ülkeleri de. Çünkü emperyalist ve zengin ülkeler bu ülkelerin çağdaşlaşmasını istemezler. (The Arabic countries couldn´t become completely modern yet. The African countries as well. Because the imperialist and rich countries don´t want that these countries become modern..)

Türk (Turkish (nation))

Türk+leş+mek

Türkleşmek (to become Turk)

Anadolu 1071´de Türkleşmeye başladı. (The Anatolia has started to become Turk in 1071.)

evcil (domestic)

evcil+leş+mek

evcilleşmek (to become tame)

Kurtları evcilleştiremeyiz. (We can´t tame the wolves.)

vahşi (wild)

vahşi+leş+mek

vahşileşmek (to become wild)

yabancı (1foreigner, stranger, 2foreign)

yabancı+laş+mak

yabancılaşmak (to estrange, to become foriegn/stranger)

Doğduğum kasabaya gelmeyeli yirmi sene oldu. Çok uzun zaman. Ev aynı ev; kendi evim. Ama bu yirmi yılda kendi evime yabancılaşmışım. (Twenty years elapsed that I haven´t come to this town. It´s a too long time. The house is the same house, it is my own house. But I have become strange from my own house in this twenty years.)

başka (other, another)

başka+laş+mak

başkalaşmak (to grow different)

farklı (different)

farklı+laş+mak

farklılaşmak (to become different)

terbiyesiz (ill-mannered)

terbiyesiz+leş+mek

terbiyesizleşmek (to become ill-mannered)

tembel (lazy)

tembel+leş+mek

tembelleşmek (to become lazy)

gerçek (1the fact, the truth, 2real)

gerçek+leş+mek

gerçekleşmek (to materialize (direct: to become real, to become fact, while it formerly wasn´t real))

Futbolcu olma hayalim hiç gerçekleşmedi. (My dream of becoming a football player has never materialized.)

We can write minimum forty more. :) Like I said, this is a very common suffix.